Why would my friend get arrested for saving a girl?

I just bailed him out. Here is the story: He was out jogging last night and saw a couple arguing. The guys hauls off and slaps her hard and even though my friend is goofy, he can fight well. I guess he said, "I'll save you" all stupid like and hit the guy twice and KO'd him. The cops came and arrested the guy and my friend even though she said he defended her and she was pressing full charges on her bf. Now my friend has to go to court next week. Whatever happened to chivalry and doing the right thing? Should we just stand by and let good people get hurt? He couldn't call the cops since he didn't have his cell on him.