I used to have a similar problem when I started riding, before my body was strong enough to post properly and I tended to post off my feet. Personally, I knew it was happening and even though I knew I was doing everything I could (riding as often as possible, working on exercises to strengthen leg and core muscles outside of riding, etc.), I was still pretty self-conscious about it.
If it were me, I'd leave it to your instructor to help her out.
She may already know she's doing it and if she is, she may be really embarrassed if you come up to her and point it out.