Tenant Advice Needed Urgently

[Christopher Bell]

Hello! I have 2 tenants that that are in a situation. 1 tenant has assaulted the other. Both tenants are on the lease. The non offender has asked the offender to leave and he has done so peacefully. As a landlord, how should I handle this? I'm fine with the current tenant staying, she had been paying 100% of the rent anyway. Do I need the person leaving to sign anything? I do have in an email that the person leaving wants to be off the lease. Do I need to do anything with the security deposit since the old tenant is staying?