Without any written proof that your former roommate was obligated, you will have a hard time collecting the last 2 months rent. Even prevailing in an action in small claims is doubtful unless the money is significant and your former roommate does not contest. Be grateful your former roommate paid without legal obligation for 10 months. Move on.
Always get agreements in writing, its just a lot easier to collect that way. You might be able to get a small claims court to order her to pay, but it will be a bit difficult and somewhat costly to initiate the proceedings (about $300), and then factor in 2-3 days of court time spread out over a few months possibly. So you decide whether it's worth your time.