...i'm in complete agreement with PMB on this one...you have to confront him with his behavior, no matter how sorry he says he is for it...especially if you guys are as close as you say you are...turning your head and pretending like everything will work and he will learn his lesson is not going to help anything, except for maybe you from dealing with a confrontational situation...
...one of the worst things that could happen from this is that she comes back to him after a while, and since he never really dealt with waht he did, thinks he can do it again if he has to...and she will have to live with the knowledge that she has to be careful of what she does or he'll hit her again...it will make a horrible life for both of them...even if they don't ever get back together, she still has to deal with that thought in all of her future relationships...
...abusive relationships are a big problem...a lot larger than one fight...i got the shit beat out of me regularly as a kid, let me assure you that it is not something you get over easily...