If you did not authorize the work, you are not liable for it: the people who authorized the work--the couple who moved out--are liable, and the company can sue them for the money; but again, you are not responsible for anything you did not have done. And no one would be responsible for their legal fee unless there was a contract or service, etc. agreement stating that if they need to go to collections or legal, they could recover the cost thereof--in the U.S., with a very few exceptions (which this situation does not fall under), each party bears its own legal bills unless there is an agreement to pay the other party's bills.Of course, they can still try suing you, even though you evidently have a good defense--there is no "prescreening" of lawsuits to ensure they are valid--and force you to spend money and time dealing with it. It may be in your interest to try to settle the matter with them for an amount you are willing to pay to avoid the potential cost of litigation. If you do settle, before paying anything, get in signed writing that the amount you is in full satisfaction of any claims.