Local housing laws vary.

First of all, if the landlord agrees with the sublet, he will have to right to sue either you or the sub-tenant, or mostly likely both, for the backrent. If the landlord does not know or does not agree, he can only go after you. Between you and the sub-tenant, if you have an agreement, you can go after him. But if you cannot locate him or you do not have an agreement, you are mostly out of luck, I believe. As for the copy of driver's license or SS card, it's just between you and the sub-tenant to decide.