The problem described was not that she wants to double tap and can't: it is that she double taps without meaning to. Telling her about pinch zoom won't change that.
The main issues are that gestures are not consistent across apps. So if I teacher her pinch gesture in one app, it might not zoom in another. If it doesn't do anything, she is most likely to get frustrated because to her, pinch gestures should zoom. I personally love gestures, but an interface that revolves around gestures might not be good for people who are already a bit afraid of technology
Pinch to zoom isn't so intuitive either. What happens when you pinch to zoom in Contacts for example. I'm guessing nothing. Of course you can tell by looking at something whether it is "pinchable" but it's not intuitive to know.
