Even the best singers can make lousy voice teachers. They need to know how to pass on to another person the skills and techniques they themselves know. They also need to know where and what to start with when they take on a student, since all singers have different voices and abilities. If your friend is enrolled in a music program at some college or university (and she would have to be in some sort of advanced program for her major), there should be voice lessons available through the school either as a class (usually open to anybody) or as individual study (which you usually have to get prior approval for from the instructor). Some schools will have singing classes geared especially toward non-singing music majors (piano students for example). She needs to check it out.

I took voice lessons too (and was in a lot of very selective groups), but other than giving some general tips, I wouldn't consider myself qualified to teach. Remember there is actually a degree to teach other people to sing (vocal pedagogy). It also helps if you know something about anatomy as it pertains to singing since there are very physical aspects of singing that need to included in any instruction for the best results and to avoid potential harm to the voice.

Meanwhile, against my own judgement, here are some websites that I've found to have an incredible amount of good information on singing. I'm only suggesting you read the articles–I'm not endorsing the programs or instructors one way or another. Since you do have experience yourself you should be able to evaluate what articles are the most helpful (you did TAKE voice lessons at least?)

Http://www.voiceteacher.com/
(check the archive)

Http://www.singwise.com/index.php

Http://www.vocalsplendor.com/

Http://www.vocalist.org/

Http://www.vocalist.org.uk/

Http://web.ku.edu/~cmed/private/wurglera…

For resources (like free sheet music)
Http://www.music-for-music-teachers.com/…

Http://artsongcentral.com/

Http://imslp.org/

Http://www.wcu.edu/6530.asp (sound files)

For modern songs, go to a commercial site like musicnotes.com, sheetmusicplus.com, or onlinesheetmusic.com as well as Amazon. These are all reliable, safe websites to buy sheet music.

Improper techniques definitely have a lot to do with a singer developing nodes. However things like too heavy a schedule with little or no rest in between, the incorrect repertoire for the voice, etc. Can also lead to nodes. Maybe a lot of singers–especially those in pop or rock music–feels that nodes are inevitable because they stubbornly refuse to take voice lessons out of a misguided fear that either they will end up sounding like "opera singers" or be less than real or "authentic" because they actually have some formal music training. Think of all the rock musicians that brag that they can't read music when you KNOW they took piano lessons since they were little kids. Of COURSE, they can read music!
One sign of vocal cord damage is a raspy voice, which is even desirable in certain styles of music. We certainly get our share of young singers on Yahoo complaining their voices are too clear and pure and they want to know how to make their voices raspy like their favorite singers.

Deliberately trying to develop nodes can lead to a permanent loss of voice or range. There are some singers after a long career that are pretty much croaking out their old hits without any of the range and power that made them famous in the first place. Even new singers like Adele and John Mayer had various vocal cord problems that forced them to quit singing and seek surgery (which cost them and everyone associated with them a lot of money). Sometimes surgeries which are usually safe and effective, can go tragically wrong. In the case of Julie Andrews (who later sued), it destroyed her beautiful singing voice that people loved so much in musicals such as "Sound of Music", even though her speaking voice sounds perfectly fine.

How to avoid nodes? First of all, you absolutely MUST find a voice teacher or vocal coach to work with you so you know what techniques are safe. You can also safely explore your voice and try new things under the guidance of an expert who can tell you if you are ready for that particular song—or if you will ever be ready for that particular song. The voice teacher should be very well trained, have musical degrees relating to voice or vocal pedagogy from credible universities, colleges, or conservatories, and have a very keen grasp on the actual physical processes that are involved in healthy singing. Then they need to be able to explain and demonstrate what they know to you so you can understand it.

Many professional singers have taken voice lessons, and furthermore, they continue taking voice lessons throughout their lives and careers to make sure they are maintaining healthy vocal habits.
Some have vocal coaches that travel with them or people they go to when they want to work on new or difficult music.

Most people can't afford that kind of intensive coaching, but on the most part, regular singing lessons can be surprisingly affordable if you are lucky enough to live in a city big enough so that there will be more than one instructor to choose from.

Nothing is foolproof. Some people just have more delicate singing equipment than others. Even well-trained opera singers like Natalie Dessay and Luciano Pavarotti developed nodes. In Pavarotti's case, it was very early in his career where he admits he was having problems catching on to how to properly sing opera and was thinking of quitting. Supposedly Dessay developed hers by going back to singing before she had fully recovered from the effects of a bad cold or infection. Then again, she was also singing dramatic coloratura roles when her voice wasn't really of that fach.
Pushing a lighter more lyric voice towards heavier repertoire can also take its toll, along with stress factors which also figured in Pavarotti's vocal problems as well.

Http://www.reference.com/browse/vocal+co…
Http://articles.chicagotribune.com/2004-…

If people wonder why I go off on rant when someone post a small child singing operatic arias in an exaggerated "big" voice, this is why. Most of those children won't be singing as adults. The vocal cords will be shot by then.

Hint: even if you do take voice lessons or have taken voice lessons in the past, make sure the minute you are having any vocal difficulties to go back to lessons, and also seek the help and opinion of an doctor–especially an ENT (ear, nose, throat specialist).

Go to the doctor already! It's been six years. You need to see an otolaryngologist or ENT (ear,nose, and throat specialist). Have the doctor examine you to determine if there has been any damage to your vocal cords. Sometimes damage can be permanent–especially if misuse or neglect continues. You won't know if this is your case until you go. The doctor can then recommend any treatment that might be necessary to help you recover.

People's voices also change as they age and grow. I don't know how "younger" was "younger", but your range might be different now than when you did a lot of performing. Plus if you did a lot of belting and "growling" as you put it–without any sort of training from someone who actually is qualified to train singers–you could have done an incredible amount of strain to your voice without realizing it until you found, as you did, that you simply couldn't sing any more. The fact that you worry that you have strained your voice–and you keep trying to push it to the max–isn't going to help you. Singing should never HURT! Pain is nature's way of telling you to please stop what you are doing.

So, go to a doctor and get yourself checked out. Then go to the best voice coach or teacher you can find–one who specializes in teaching your music style, but one that can rehabilitate problem voices. This teacher still should have an university or conservatory degree relating to singing, voice, vocal performance, vocal pedagogy, etc.(any or all)–and have a terrific understanding of the actual physical processes that go into producing sound in a healthy, natural, and beautiful way.

Good luck. Below are some articles that go into possible problems. They are meant for information, and not an endorsement of any particular teacher, singing program, or physician. Find a local doctor–then find a local teacher.