Legal Advice Needed: One member of the lease gave 30 day notice after 13 months... remaining tenant moved out owing rent.

[Wendy Brown]

18 months ago I rented to an unmarried semi-retired couple. They both signed the one year lease. 13 months into the lease he needs to move out of state to care for his elderly Mother. He gives a 30 day notice and tells us he is giving his share of the security deposit to the remaining tenant. She is the lower income/asset earner and in a short time it is determined that she cannot afford to live here. She willingly moves out but she didn't pay the last month of rent believing like so many people that it is first and last months rent! ugh!! So because of an especially bad snowstorm her exit plans were delayed by 4 days and what with the final gas bill and cost to fill water softener she owes us an additional $700. Because her partner was originally part of the lease can he be held responsible?