2 tenants split sec dep, 1 moved out, 1 stayed, wants 1/2 sec dep

[Jon S.]

I rent to students, they rent as 2 students on one lease, and they put down first, last and security. At the end of the lease, one student gave a 60 day notice and moved out, while the other student decided to stay for an extra month, as month to month. I'm not sure how to handle the return of the security deposit, as the tenant who moved out wants her portion (1/2) of the security deposit now. Suggestions? What is the normal way to handle this? Thanks!!