One roommate wants to move out. Both roommates are joint tenants.

[Ellie Narie]

I'm renting out a 2 bedroom apartment to two people. They're on the agreement as joint tenants. They had a disagreement, and now one person wants to move out, and wants half of the security deposit back ("their" portion). On the rental agreement, it's written as though they both paid the security deposit jointly, since they agreed to rent out this apartment together. 

The other person wants to stay and find a new roommate. 

What should I do in this situation? Do they both need to move out before I return the security deposit? Should I just do a walk-through and return the moving tenant half of the security deposit minus deductions?