I'm a girl, and I say have her get it done. The reason I say this is, it's her decision, if you disagree she's going to be pissed, if you agree she's going to wonder what size she should get. So either way your going to be screwed. Just agree with her. In the end, she'll have the implants and pain, and you won't have to hear about that shit anymore.
The next thing is she'll want a tummy tuck, don't agree with that. If so your really fucked.
