Apartment Damages

I was wondering on what a apartment complex can do for damages that were done by one of the roommates dogs and her boyfriend. The carpet was ruined by the dog and the boyfriend put a hole in the wall one night. We never got our deposit back and now the credit collectors keep calling saying my fiancée still owes $500 on top of not getting the deposit back. Their lease was up a couple months ago and she never got a letter that we know of she has moved a couple times since then but she has changed her address at the post office both times she moved. I was wondering if there was anything that could be done so she doesn't have to pay for something she had no control over. Thank you.