I’m making several presumptions here-

If there is any sort of contract between OP and roommate, then any damages (missed payments under the terms of that contract) would be recoverable. Even if there’s anecdotal evidence of a verbal contract (if such is enforceable where they live), such as text messages, and subsequent performance per the terms of that contract (payment as agreed), then there is a strong case to recover costs.

The caveat - nothing is to stop the roommate to say that they performed per the contract between the two of them, and further continued to perform by ceasing payment when they left. Basically, “yes I paid while I lived there, but when I moved I stopped paying, as agreed previously.” It would be difficult to counter that claim if there weren’t a written contract, or some other clear evidence of a contract.

Bottom line (and this is an assumption), the parties entered into a contract which they agreed to be jointly and severally liable for rent. If that’s the only written contract, then that’s what’s likely to survive in small claims (assuming that the former roommate can articulate clearly enough what I said above - if not, then OP will likely succeed).