Well, it depends who the guy is, who she is, whether I care about either of them or not, and whether I actually know them. Oh yeah, and it depends on what preceded the violence.
If your asking if I saw a man beating up a woman, and I don't know either of them, and I don't know what preceded the violence, then honestly it's pretty egocentric to even think of whether I would forgive the man or not -- it's an irrelevant question because it has nothing to do with me. I definitely wouldn't approve of it, because it's violence, and I am against violence, and it really doesn't matter whether the aggressor is a man or a woman. Violence is violence.