Joint Tenancy

Tenancy Types and Management

I have a couple whose tenancy is subject to a joint tenancy agreement.
Their relationship broke down at the beginning of the tenancy - as far as I'm aware the girl has never been in the property. Also she has never contributed to bills or rent.
She is now demanding access to the property.
Does anyone know what her rights are and those of the other tenant and mine as the landlord?
Can one of the tenants end the agreement without the consent of the other?
Can she demand half the deposit is paid to her?

Any info here would be gratefully received
Thanks
Sky