Tenant problems: Each wants the other to move out but they won't!

[Joe Berger]

I have a couple who has been renting from me for a few months now.  The lease was approved based off of her high income, but they are both on the lease.  Now things are sour in the relationship and she doesn't want to live with him anymore.  She contacted me to see what her options are as far as the lease goes and I said she could take over the entire lease, but I do not feel comfortable for him to, even with a co-signer.  She pays the rent on time and is responsible, but he is on the lease and threatens that he won't leave because of it (even though she is paying the full rent). 

Is there a solution for her other than them working it out between themselves (which doesn't seem likely)?  Is there a way to remove him from the lease without his agreement?

I'd like to keep the female tenant, and I think she might stay in the house longer term.  I realize it's not my problem as landlord (unless they stop paying rent), but how have other tenants resolved this sort of situation.  Any advice?  Thanks in advance!