If I understand the facts correctly, yes, you're responsible for the rent, but so is the departing tenant. If the tenant has already moved out, you will probably need to file a small claims action to collect. You can try to get your landlord to help you collect by withhold that tenant's share of rent, but it's just as likely that your landlord will try to evict you for nonpayment of rent as he is to go after the departing tenant. If you withhold rent, you will want to hold on to it so that you can pay it to your landlord or into court if you get a 72-hour termination notice. It may be worth hiring an attorney to help with negotiations between the parties.
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