Well it sounds to me like she is inviting the conversation. Next time she compliments you on your hair just tell her what you do, or you can say "Oh I think your hair would be just as long as mine if you stopped heat styling, I know a lot of really fun ways to style hair without heat do you want to try them some time?"
I think if she invites the advice by saying, "I wish my hair could be as long as yours..." then you can give her whatever advice you feel is appropriate.
I wouldn't make a big deal out of it, though. It's not an intervention, it's just hair advice.
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