Etiquette on Couple Separating Right Before Invitations

Okay so my FH is friends with a guy and his wife. We've gone on vacations and stuff with them and they've been great to know. We sent out the save the dates to them in January. ... Well fast forward to last month when they abruptly separated. The guy is now dating someone married and also separated. FH is definitely more friends with the guy. So for his invitation what do we do? We obviously can't invite his wife, but it really bothers me to give him a plus one and have him bring his married mistress to our wedding because I liked his wife and it feels wrong since everyone's still married. Can we get away with just sending him an invitation with no plus one? What to do?