It's very easy. You do it when you want to fuck your girl's female friend behind your girlfriend's back. And you aren't insecure to the point you think your girlfriend will cheat on you the minute you leave the room.idk bro, they might be swingers and we all know Andy is a cuck, i cant think of a logical situation where i would ever take one of my girls female friends home and leave my girl at the party.
or don't care if she does. For all we know their relationship might be just arranged business thing for Bianca's boyfriend and he isn't all that invested in her)