Would you tell the girlfriend?

My brother and I live together and he has a girlfriend. My brother's best friend and him randomly had a failing out because his friend got drunk and said some shitty things to him. They've been friends for so long and I know it hurt my brother's feelings. 4 days ago my brother told me his best friend called him wasted asking if he could come get him and asked me if he could crash here. I said sure. He picked him up and he threw up on our floor so I went to get a trashcan and came downstairs to him apologizing to my brother for all the things he said and then went and kissed my brother flat on the lips. I walked back upstairs. I asked my brother about it later and it seems his friend was upset because he was harboring feelings for my brother and that's what made him lash out. I asked was he gonna tell his girlfriend what happened but my brother said he's confused. About WHAT? He already has a GIRLFRIEND! I told him he needs to tell his girlfriend his MALE friend KISSED him. And he hasn't yet. We have Christmas with them TOMORROW! I would want to know if I was his girlfriend but my boyfriend is telling me to mind my business especially since me and my brother live together and let my brother figure it out. I don't understand what there is to figure out. He has a girlfriend. That's it! Would you tell your brother's girlfriend?