Hmmm- this is a tough one.
Do you know which sort of yoga she does? If someone were to buy me an Ashtanga present it wouldn't be a whole lot of use to me because I do Iyengar yoga, and the disciplines are really very different. If you know which sort of yoga she does then I'm sure that there are bits of kit (everything from belts, mats and bolsters to trestlers and back stretchers) which make good presents.
You could always get her a course of ayurvedic massage or similar if you don't know the details- there's an ayurvedic therapist working at the new Iyengar centre in Walkley.
Insecurities are about as useful as putting the pin back in the grenade.