Removing one of two tenants from a lease

Hi all, 

My tenants were a couple when they signed the lease. Now they are not. The bf is planning on staying, and would like the gf off the lease. I have no problem with this (although they have not been the greatest tenants). What, specifically, should I do? Re-create the lease and have him sign the new one? Are there any legal issues with removing someone from a lease, or drafting a whole new lease before the end of the previous lease? I assume the gf will want to be off the lease, but can I just go ahead and do that? I just don't want anything to come back and bite me just because I don't have the experience yet. 

He also wants to change the locks because she did not return the key. I have no problem with that either, but is there any legal reasons why I should do this instead of just telling him to do it and give me a copy of the key?

Thanks!

Jason