depends on what you have in your contract. We rent to quite a few college students, and mixed room mate
our contract has this
1. IF THE UNIT IS OCCUPIED BY MORE THAN ONE PERSON, TENANT’S ARE INDIVIDUALLY, JOINTLY, AND SEVERALLY, LIABLE FOR THE RENT, PROVISIONS, TERMS AND CONDITIONS AS STATED HERIN. If one tenant vacates, the remaining tenant(s) may be required to re-qualify within 30 days of the change in occupancy. The security deposit, and last months rent if applicable, shall stay with the unit until final termination of all tenants.
Being that you wrote a NEW contract- it may mean that you have to refund the deposit from the OLD contract.....
It is not the landlords problem to determine WHO caused the damages....that is between the tenants.
What we do is to renew the contract with an addendum adding or subtracting tenants. Did the one who moved send you a notice? If not, they have not removed themselves ( unless you have a term lease)