what do you do? how do you react? do you do anything about it or just let it slide as it is basically between the two of them?

basically, i've just spent the evening trying to console my best friend and had her cry on my shoulder as her boyfriend is being an arse, and i want to physically hurt him, or at least have serious words with him. but at the same time i know that it's not really acceptable to do that, or at least it's not the done thing, so i don't quite know how to deal with the whole situation.

anecdotes and/or opinions below, please.