I'm content in knowing that if some asshole did that to my wife she would have elbowed him accross the throat several times, "to ensure a peaceful night sleep"
then dealt with the repurcussions in the morning.
on the other note you should confront this mother fucker because HE obviously has absolutely no respect or fear of you. if he had any respect or fear of you he wouldn't have even spoken with her let alone tried to slither into her bed. Oh wait he DID do that, SEEE?
I suggest striding directly up to him and without preamble of any kind punching him directly in the throat, step through shoving him onto his back, headbutting him in the nose.... whatever seems to fit best for you, then immediately continuing with WHO THE FUCK DO YOU THINK YOU ARRRRRRE!!!!! If ever there was an appropriate time to come un-fucking-hinged at another man this is it. Over stepped his boundaries?!?! WTF overstepping his boundaries is stepping into a room where an attached girl or girls are sleeping, setting up camp in a foreign country is what he did. You
should go cro mag on him.
This is not a time for tea and crumpets or even a slap across the face, it's a time for war, are you ready for war? How fiery is that heart of yours? Or is she not actually worth it? If so, fight to defend her assaulted honor.
I trust you haven't seen him at all yet, if you have and just given him dirty looks the initiative is lost. It can be days after the event but the first thing you have to do when you see him for the first time is physically confront him even if it gets you in shit, because not only is that right but it is also defensible.