I think it's safe to say being shot in the boobs equates to us guys being shot in the dick. Finding a small enough plate carrier would be a bit of an issue if she's really slim, you'd really have to bring her somewhere to have her try it on. Also a full face mask would probably be most beneficial.
As for gun selection, I've brought smallish girls to the field before, and they didn't mind using m4s, AKs or what not... the last girl I brought to the field thought my p90 was too cute to use. But just like the guys, once you pick it up and shoot a bit, it feels so nice easy to move around with.