Who is responsible for security deposits when 1 roommate moves out and the other stays?

The answer to your question largely depends on the wording of your lease and how the landlord collected the security deposit from the tenants. If your lease has language stating that the tenants are "joint and severally liable" for all provisions of the lease, then you may have little recourse against the landlord. This provision means that you and your roommates are jointly, as well as individually, responsible for the rent, obligations under the lease, and any damages to the apartment. This phrase is also used by landlords to mean that the security deposit is held "jointly and severally" and that they are not under an obligation to return a "portion" of the deposit when one tenant moves out.

However, if your lease does not have this language and/or the landlord collected your security deposits separately, providing you each with separate security deposit receipts, and represented that he would return each tenant's portion of the security deposit when that particular tenant left the apartment, then you might have had a claim to get your portion of the deposit back when you moved out (assuming that at that time, there was no damage to the apartment warranting a withholding of the deposit).

Further, assuming that the landlord lawfully withheld the security deposit for damage to the apartment (and the damage was caused by the tenants and was not the result of 'normal wear and tear'), then you have no recourse against the landlord for the return of your security deposit. However, you may have a cause of action in Small Claims Court (assuming your portion of the security deposit was less than $2,000) against your sub-lessor (the person who took your place in the apartment) if you can prove that this person: (1) did not pay a security deposit; (2) caused damage to the apartment and (3) but for this damage to the apartment, you would have had your portion of the security deposit returned by the landlord.