I just found out the girl who is leading me on has been staying at her exes place. What should I do?

I started "dating" a girl back in april and recently she cut off all sex but wants to be friends. She has a key to my apartment and wouldnt give it back. I tonight found out she was spending the night at her exes house after she said she was hanging out with her other friend. I told her I want my key back before she goes to work at 10 am. What should I do when she brings it back?