Locked key inside house?

Hello, i am dog sitting for my neighbor and he has those locks that can be locked from the inside on the handle and when they are closed they cant be opened. Well, being that those locks are design flaws, i left the key on his table on the way out. Now the key is inside the apartment where i cannot get to it. I tried that whole credit card trick, didnt work. My next step should be to ask him if there is a spare key or ask his apartment neighbors. Anything else i can do?