Yeah but they were broken up. So he didn't cheat.
If the girlfriend likes the girl then there shouldn't be a problem with supervised visitation in a few months time when she has had time to settle down.
If she didn't like the girl to start with then basically choose one or the other.
But honestly when you got back with your GF you would have had to have known this day would come!
You have to ask yourself what's more important a rocky relationship or a drunken sex buddy.