FI had his Bparty this Sat! It started out well with the guys going to the gun range, then taking the party over to FBIL's house to drink and watch the fight. FBIL and his GF are currently in a rocky state, where she's just living there for now until she gets back on her feet and is able to move out. So at around 3am FBIL's GF comes home from work, and somewhere in the course of the time she's there FI's BM slaps her on the butt 2-3 times. Now although FBIL and her are contemplating a break up, FBIL is extremely jealous and now super p'd off.
Now here's my dilemma: FI's BM is engaged to my bestie. She already blew up after hearing FBIL's GF in the background the next morning. Do I add to it by telling her what happened? or do I avoid causing an argument and tension at my wedding?
Help!