Hmm... I think she may be a bit too obsessive-compulsive about hair. Straight hair must surely appeal to obs-comp types because at least you know where every strand is, so to speak. Curly hair is always going to be more unruly no matter how healthy it is. Could she therefore be mistaking 'healthy' with 'unruly'? Like the parent who freaks out because their kid has just spontaneously decided to do paint-hand-print artwork while sitting on the living room carpet?
If that's the case then I'm prepared to hazard a guess that she gets obsessive about other things, too. Would I be right about that? If so then she has to manage her anxiety about perfection all the time and she just happens to be spilling it out onto you about your hair.
In your situation I think I'd calmly say to her that you're happy with your hair and that actually, you're training it to be healthy.
How extreme her obsessiveness is is best guessed by you or herself, but it may be an interesting thought exercise for her to see you, her friend, with 'not-perfect' hair and to grow to accept it as much as you do.