One person wants to leave

Tenancy Types and Management

I have two girls who signed a joint assured tenancy agreement for 6 months on the 7th November, unfortunately they found they could not live together despite being friends most of their lives as they were arguing all the time. One of the girls has left and has asked for her name to be taken off the tenancy agreement, in principal I would have no objection providing the other one is prepared to take the responsibility but legally how do I stand. Does the duo have to stay as is for the 6 months? The rent guarantor is the one who is stayings mother.