tread on her foot, poke her in the eye and push her backwards, (only take your foot off her foot or she wont fall).
Seriously gal its a load of bull and this person is probably just looking for an excuse to hit you. I can remember exactly the same scenario when I was around 15 with two sisters called Janice and Jackie Brown, Janice was tall and very pretty and her sister Jackie was like a sumo wrestler. For some reason they took a disliking to me and did the same trip, one of them said I'd been talking about the other one (which I definitely hadn't cos for one I was scared ********* of the butch sister). Anyway, I was told to be at such and such a place and I bravely went along (feeling terrified but not wanting to show it by not turning up). Nobody else turned up though and it all blew over.