How to handle security deposit when one roommate moved out?

[Lisa Craig]

I have a condo I rent out in Michigan. I rented it to 2 roommates and the lease is up at the end of July.  Back in February they were not getting along and one of the roommates just up and moved out without notice to the other roommate or myself.  Neither I nor the remaining roommate have not heard from this roommate since.  The one remaining roommate has paid the full rent each month.  The lease is now ending at the end of July and I have the full security deposit still.  When they moved in they each paid me 1/2 of the security deposit even though no where in the lease did it state they each needed to pay half, just what the total security deposit was. 

I am wondering how to give back the security deposit.  Should I just give it all back to the remaining tenant?  My thought is that she has paid the rent since the roommate left so she should get the full deposit.  There is no unpaid rent so I feel that I can't keep the half of the roommate that left without notice.  Also, the roommate that left gave no notice and no contact information so I wouldn't even know how to return the security deposit if I needed to.  

Thoughts?

Thanks!!