Under PA law, what are landlord's rights when one tenant breaks the lease and the other tenant stays.

Both tenants have signed the lease, put up half of security deposit and split rent. After 4 months, one tenant moved out and wants name off of lease and return of security. I am willing to do that once another renter is signed on in her place. Also, I understand she is still responsible for rent payments until she is off of lease. She claims that the remaining tenant will drag his feet to find a new roommate just to cause her problems. What are my options as landlord. I depend on that rent payment to keep up the bldg.