If its just your name on the lease, guess who's cheating ass would be gone. Hope that is the case op. sorry man

Maybe it's a joke? hopefully she wouldn't do you that dirty.

Kick her out. But before you do that, make a nice dinner and then get on one knee. Have her open the box and put a note inside saying "You slut I hope you get Aids from your little buddy. Your things are scattered in the dump. Have a nice day." and when she's gone send a message saying "p.s, I changed the locks."

Just ask if you can watch them making out. Or even join them.

Better find out if it's true before you do anything irrational. Her girlfriend might not like you very much and wanted to break you guys up.

Firstly, as others have said, check the situation, as someone saying something does not automatically mean it is true.

If it's your name on the lease, what's the problem? throw your (ex?) GF a bag party. buy a roll of garbage bags, bag up all her crap, change the locks and text her saying that she's homeless now.

tell her it's all good the more the merrier

I assume the issue with the lease is that, if she is cheating, she will leave and you'll be responsible for paying the rent by yourself. Hopefully, the apartment's not a one bedroom and you can get a kickass roommate.

So? Kick her ass out and find a roommate!