Don't get me wrong, bitches be crazy and will do **** like that for the reason you mentioned, but a punch is escalated force and the law, indeed society, frowns on such actions as self defence.
If I found myself in such a fucked up situation, I'd take off my own shoe and chase her down, using it to spank her while calling her a "naughty girl that needs to be punished". This would result in her performing willing and enthusiastic fellatio on me and asking her friend to lick my balls before I **** them both against the truck in front of everyone. But that's how I roll. Your homie seems like a misogynist and a woman hater.
First I would suggest learning a few more adjectives besides homie, **** and ************.
Second understand that self defense is a matter of first avoiding a dangerous situation, second to defuse it and third to survive it.
First your friend should have not been acting a drunken fool as has already been pointed out. This would have allowed him to avoid the entire mess and not get his ass booted from the university.
Second when slapped with a fucking slipper he should have manned up and taken it since he was the one to initiate contact and star the altercation, he wasn't really in danger or in fear for his life from a shoe right? Again he had the chance to walk away a little embarassed but otherwise fine.
Third he should't have let fly the cross. Unless she caused him bodily harm or was armed with an actual weapon he had no good reason to slug her. This is yet a third chance to avoid the mess but he failed again. Thats three strikes and you're out.
First off if you never been to Bachanal, STFU!
Second when you decide to assualt someone, regardless of size or gender, you can't be upset when the other person fucks you up, period. The only thing that I see would make this in the wrong for the guy is that he did do something physically offensive to the girl. Which is hard to believe at a frozzy rub up.
Of course it wouldn't end in a three way. I'd make her friend wait until later.