“Lucky Jim, I’ve got some people coming over for dinner but one of them is bringing their partner who I don’t really like – what do I do? “
Suck it up, lad!
Seriously, you’re the fucking host. That’s how hospitality works the galaxy over… except of course with the famous Feuding Clans of Libertaria. (With those fucking lunatics, an invitation to dine is really a challenge. Politeness demands that they let some of you escape, and that you not kill too many of them in doing so. Don’t ever take a counter insurgency gig on that backworld, especially if it specifies “hearts and minds.” However, I can tell you that they do a really nice spiced “potato” dish, so the trick is to turn up after the fight, but soon enough that the food is warm.)
So as host you have to make this obnoxious fucker welcome. Ask questions, do active fucking listening. However, as host you also have to protect your other guests from ashattery. So be ready with phrases to defuse and derail, and serve them with a fucking smile. “Moving on briskly… LOL too much information! …Where did that come from? … Let’s pretend you didn’t say that… Seriously stop treating my guests like shit and get out of my fucking home right now.”
Long term you may have a problem because in choosing a partner, your mate is choosing which friends to keep. If they’re shacked up, then I’m afraid you just lost a friend. If it’s an ongoing relationship… well, the sex must be good, so perhaps they have their hormone blinkers on. Sooner or later you have to let them know, because by the time they work it out for themselves, it’ll be too late to act and you’ll be deeper into the friendship, and they’ll be committed to the relationship.
Mates don’t let mates shack up with bad people.