"Why can't you be as good looking as your mate?" Uncalled for and harsh. I'm not one for negs usually but in this scenario i'd give as good as got. SJ you are right, if this was the other way around all hell would have broken loose the girl would most likely take offence as would all her friends in the group. As men why should we stand their and take it? Some would argue be non-responsive, and in most cases I would agree but when people say something that you consider below the belt and unacceptable, call that shit out.

Don't let people think they can say what they like and get away with it. She's looking for a reaction, most likely a defensive reaction, instead match her with an equally offensive reaction.

I'd have gone for;

"Why can't you be as good looking as your mate?"

Response: 'Why can't you just shut your mouth and let your other lips do the talking?'

Said in a cool and composed manner.