My daughter rents a apartment and she was the Co leasee on the lease her roommate who was the main leasee broke the lease and moved who is liable ?

The roommate contacted the apartment managers and they transfered my daughter's security to a new unit for the roommate which shouldn't be legal there was also a 2 month gap from when roommate rented the new unit my daughter has low income and would of never been approved for the apartment herself and it seems the roommate is off the hook and my daughter now has a $1,700 debt ??? Any help would be greatly appreciated Thanks...