Not everyone can take a joke the same way. I don't like being talked down to or anything like that but if someone was to have said that to me I could laugh it off. As for being justified for slapping her, if you done it as soon as she bit down I think it was probably more of a reflex than a violent slap. Maybe next time he has his thing in some girls mouth he should set back and enjoy it instead of trying to make jokes.
As for an apology they kind of both owe an apology to the other one.