My daughter and her boyfriend signed a lease together in California to rent an apartment. It has only been 4 months of a more than 12-month lease and the boyfriend is planning to move back to his home state because he is not happy. It is too expensive for my daughter to keep by herself and difficult to find a roommate because it is a loft-type and the bedroom area is open to the living area. She says she can sublet but that makes me nervous because I don't know if she can be held responsible for anything the new tenants break, etc. What are the options for holding the boyfriend to his portion of the lease?

Sue him for his half... or convince the landlord to allow the lease to be terminated early. And sue him for any additional costs incurred.

You are right about subletting the unit... they answer to her... but she answers to the property owner and she assumes all responsibility.