A little knowledge is a dangerous thing. (Here we go, yet again, with people who don’t know what they’re talking about answering legal questions. Sorry if I seem testy, but this has happened 87 times before, and it gets tiring spending more time correcting misinformation than actually saying something useful. Honestly, the annoyance level of this is getting as bad as the missing dollar and gry threads.)
What t-bonham and herman (who gathered his so-called information “by watching TV!”) are misdescribing is the duty of a plaintiff to mitigate his damages. (If someone breaches a contract that you have with him, you can’t stand idle and let your damages accumulate. You have to take reasonable steps to mitigate, or limit your damages. In a lease setting, this means that a landlord can’t just let an apartment stay vacant after a tenant moves out halfway though a lease term. He has to take some reasonable steps to re-let the apartment. That duty (of the landlord) doesn’t kick in here. There’s still a tenant, and he therefore can’t relet. If both moved out, it would kick in, but that’s a foolish step for the OP to take. If the landlord can’t relet (and nothing forces him to relet this apartment first, if he has others that are vacant), OP is on the hook for the whole unpaid rent.
Here, two tenants apparently signed a lease, making each of them jointly responsible for the entire rent. One vacated early, and refuses to pay his share. The other is on the hook. t-b and herman claim that this means that the other tenant must breach his lease contact, too. t-b and herman are wrong. That would just make the OP a defendant, and ultimately a judgment debtor. The duty to mitigate never forces you to breach a contract. t-bonham’s second thought is closer to the mark. The OP might be required to mitigate by trying to find a replacement roommate.
Essentially, Roommate A and Roommate B have an oral contract, which might be considered a partnership agreement, to rent the apartment together and share the costs. In my opinion, the OP can sue in small claims for breach of this agreement.
The OP asks some good questions here. A court might make you wait until the end of the lease to assess full damages. (Leases have acceleration clauses to deal with this problem. Your oral contract with roomie doesn’t.) Also, collecting any judgment that you may get may be difficult. Trusts are often set up specifically to make the trust assets immune from creditors. Anything that’s actually paid to him is fair game, though.
Usual disclaimers. While IAAL, I’m not one in your state. I’m not your lawyer, and you’re not my client. This response provides general information, and is not intended to be legal advice to be relied upon in your specific situation. See a lawyer licensed in your state for that.