Option 1 : You could upgrade your apartment into two-bedroom apartment in the same complex and designate one for the lovers. But I would keep the lease it in your name or your friend's name exclusively.

Option 2 : You could continue to bear the inconvenient truth until the lease is over by shifting everything from your hats to the socks into a closet outside of the love-nest. If that's the case, Romeo should be paying premium share of the apartment rent. After all, it will still turn out to be cheaper than a motel room  for him.

Option 3 : Check if he has any inclination for him and his girlfriend to move into their own little private shack. If positive, the news itself will be an early Christmas gift to most of the parties involved in this dilemma.

I am sure that are more ways if you brain-storm.

Good luck now.