if your friend finds it offensive then they might want to bring it to the person's attention. I would find it awkward in public but everyone is different. I don't believe I would answer to that in public because I respect myself and find that a bit off for me, but if you find it offensive then maybe you might want to talk to your friend about it and see where she stands? if she has nothing against it, then ignore it, I hear worse things in public.
Depends on the girl and the relationship. If some guy at the bar shouted out such thing to me, he'd got punched very hard. But if my best friend said that I wouldn't mind. I'd knew it's a joke and he doesn't meant to offend me.
Depends on the person. It's hard to generalize and say every girl would find that offensive. I can joke like that with my husband, sure. But if any guy outside just said that to me, he'd likely get a scowl.
If your friend's girlfriend is fine with it, so be it. If she's offended by it, I'm sure she's a big girl and can say so.
It depends on the girl. Maybe she wants her boyfriend to think of her as more than just a girl with a nice ass.
Personally I wouldn't want somebody talking to me like that, and I definitely wouldn't see it as a compliment. There's no need for it. This boyfriend should act like more of a gentleman towards his girl... In my opinion
some girls don't mind some do. my friend talks like that to his girlfriend too, not my cup of tea personally, but she doesn't mind and thinks it's funny. but he does it in a manner which he obviously does not intend harm.
Are you asking because you want to know if you should address girls like this?
The answer is no, unless you and she have some kind of understanding of an inside joke that makes this acceptable, and she is okay with it. Even then, it might be a little offensive to others who overhear.
He may think he is complimenting her because he thinks she is hot. I still think it should be reserved for when you are alone instead of shouting it for everyone to hear. Obviously, she is not okay with it.