Question: Are you (The GM) also the host? There use to be a set of "Role-Player's Etiquette" that covered this kind of thing, but I guess with the way things are these days, no one really knows that it exist anymore.
My advice, you (the gm) and the host (If not you) need to meet with the two causing the issues privately. I understand that it is mainly the boy-friend, but as the girl-friend is involved, she needs to be approached as well. Just sit down with them and have a friendly chat. Explain how their abandonment of the group in the middle of the sessions is putting a strain on the group as a whole. That you understand that everyone has a limit to how much role-playing that they can deal with at one time, but that you need them (individually) to commit to either staying for the full session, or consider backing out. Be nice, be polite, but be firm. Rather than saying, "You're not welcome in my game (One of the GM's rights and responsibilities) or "I don't want you in my home (One of the Host's rights and responsibilities), put it into their court and make them either back out of the game (gracefully or not) or commit to the sessions that they have already committed to.
I've seen parties fall apart because players get frustrated by situations like this, and how you handle it is very important. Never, ever do it during a ssession, and never, ever do it in front of the other players. As GM, you have the responsibility to listen to your players and act in their best interest; a responsibility to provide the group with a fun and enjoyable experience. This includes talking to players who are causing problems. As a host, you have the right to ask anyone causing disruption to your own enjoyment to leave and a responsibility to see that your guest are enjoying their time in your home. It doesn't have to be a big deal, but it does have to be dealt with. So, again, keep it friendly, keep it polite, but explain what's going on in private and let them make that decision. If they stay and the difficulty continues, then you'll have to ask them to drop. If the girl-friend wants to continue to play and the boy-friend doesn't, offer to make sure that she has a place in the game and a safe way home after the game (If she needs it).
Hope it helps.