I spent years in boxing as a kid. I figured I'd end up in fights at some point, but I found out that being confident and using discretion about what you do and where you go makes all the difference. The self-defense classes will give her the confidence to enjoy her life and avoid these problems altogether.
That said, I've got two daughters, and I'd give them whatever kind of most effective weapon is legal. If it's mace, then that. If it's a gun, then that.