So whose name is the new apartment in ? If the new apartment is in his name he can tell you to move out and you have to move out. Even if the apartment is in both of your name and if his girlfriend is staying with him and in his room; then there is nothing you can do about it. If you go and complain that to your apartment; they will not do anything (worst case scenario, they might terminate his lease for non compliance but that is very unlikely). But if the lease stated clearly that you can't have more than one guest for 40 days, it's up to you to make sure you stay in compliance (if the lease is in your name too).