What do I do if my steroid raging friend punches a girl?

I was out this one night watching most of my friends running around trying to pick girls.  Most people at this place are seemingly in a good mood except for this one girl who apparently was about to 'mess with the wrong guy'. My steroided out friend (we will just call him Frank) goes over to this girl and does that little playful gesture where you point down on someones shirt (as if you were pointing out a stain).  The whole joke (as most of you know from Kindergarten) is to hopefully get the person to 'believe' that there is in fact a stain (or something worth looking at).  Then....As SOON AS THEY LOOK DOWN, you then complete the joke by playfully flicking their face (not hard but in a way that you let them know you got one over on them). Well 'Frank' successfully does this to this random girl.  Now I have watching this playful exercise executed 100s of times....and NEVER have I seen a girl react the way she did! She froze, as a look of absolute disgust took over her face.  She then took the drink she was holding and threw it at Frank, and then walked off!  Now Frank (apparently fresh off the juice aka steroids) sat there and stewed for a few seconds and then charged after the girl. What I saw next was another 'first'.  Frank cocked his arm all the way back, and then proceeded to put all of his 350 pound bench pressing strength into punching this 120 pound girl.  I was shocked!  This was a kid that I had been friends with since 6th grade and wasn't aware that he was capable of such insane behavior.  Sure, she overreacted to him and perhaps deserved a little retaliation, but this was taking it to a whole other level!   Frank must have realized what he did because 3 seconds later he was out the door running.  I haven't heard from Frank since, several weeks have passed.  How would you handle this situation?