Read Nicki Doane's resonse:
There is always going to be that student or colleague who thinks they have all the answers. I find it best to be firm and clear in telling them that if I want their opinion or feedback, I will ask for it. If they do not like your classes or style, then they don't have to come.
Your student sounds like a busybody who needs to focus on her own practice. The practice of yoga is about looking within and seeing ourselves as we truly are. If she is constantly looking outward at you, then she is missing the point. The way to get her to concentrate on her own yoga is to ask her to. When you approach your friend, ask her if she can stay after class or perhaps meet you at an alternate time for tea so you can talk privately. Tell her that while you value her opinion, you will ask her for it when you want feedback. Most importantly, let her know you value her friendship and want to keep that separate from teaching. Be clear that you don't want the teaching to get in the way!
Don't fall prey to feeling bad about yourself and your teaching because of unsolicited advice. However, if you find that after you've thought about what she's said for a while, you feel like it rings true, then maybe you could ask her for some of her time and get more feedback. If I were you and thought that some of what she said might be correct, then I would go to my own teacher or mentor and ask them about it.
Yoga is about learning who we are and also about learning to love ourselves. Don't lose confidence over someone else's opinion of you. Maybe you could suggest that it sounds like she wants to be a teacher herself and, if so, she should go out and take a teacher training course. Perhaps you could recommend one!