They stole items from the house - that is a police matter, and your mother should contact the police and file a report. If your mom had insurance, the insurance might cover the loss of the items, so she should review the policy to see how to make a claim or to talk with her insurance agent. As for the rent, you can see if they filed a change of address notice at the post office.
If she had a contract she could take them to small claims court, all she has to do is file,and the court system will eventually find them. If she rented it with no contract, no deposit, then its a live and learn situation.
small claims court seems the best way to go and also filing a police report. best of luck