I'm glad you last saw your FIL on a high, and he enjoyed his day. I'm disgusted by your XAH's inability to pull it together even for a short time when his mother needs him.
I know this is an emotional time for the family, but please resist running around after them when they could just as easily catch a cab, grab some lunch, work out how to book online etc. Your XAH may well be thinking he can drink because you'll be there to pick up the pieces. MIL can seek advice from many professionals, and it would be a good idea for you to be kind, but encourage her to do that.