I just think someone is spoiling for a fight...
I don't condone violence, although I understand the need for it...occasionally.
I think if he were routinely beating on her it would be a different story. As it stands and from what I gathered, it was a one time incident. If they're reasonable individuals, this will probably be the only time it ever happens. I'd talk to him, I'd explain what happens if it continues, give the guy a pat on the back and move on.