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A landlord is actually not required to return a security deposit until all tenants on the lease have moved out. So you do not have to return half the deposit now - you can wait and account for it within 21 days after the second tenant moves out. Usually what happens in this scenario is that the tenant who is staying pays the tenant who is leaving for half the deposit (or a new roommate who comes in gives a security deposit, which goes to the tenant moving out).
California law doesn't allow a landlord to request a new security deposit in the middle of the lease (because a landlord isn't supposed to touch a deposit until the tenants leave). What you can do is sign a new lease with the remaining tenant for the total rent that says she's going to be the sole tenant going forward and that stating the full deposit amount. Then you can note that you're applying half the existing deposit to the new deposit and returning the rest to the old tenant. Everyone should sign the paperwork.