Recently an old roommate has moved out and a friend of hers took over her part of the lease. Since the lease was out in December and he moved in September, he didn't sign the lease. His dad paid his part of the rent each month. However, we had to pay the power bill but he has yet to put anything on it. He moved out in February and still owes up close to 500 dollars. Is there any way to sue him for this even if he didn't sign the lease?

