Recovery of Pre-Paid Last Month's Rent

My question involves landlord-tenant law in the State of: MA

My roommate and I pre-paid last month's rent (August 2013) upon moving into our apartment. My roommate moved out at the end of May, and I had originally planned to move out in June but instead moved out at the beginning of August. While I was deciding whether to stay in the apartment for the summer, I had lined up a subletter for my room, with whom I had signed a contract stating that she would reimburse me for my portion of the August rent we had already paid.
Once I decided that I would stay for June & July and lined up a subletter for August, and I informed my roommate via text that her room was "taken" for the summer when she asked me if she needed to find a subletter. My roommate told me, via text, that she wanted me to "get money from the subletters and pay rent that way, without [her] involvement", and I agreed to this via text saying "yeah that was the plan". However, she never explicitly asked me to ensure that she was reimbursed for the rent she had already paid, and didn't take any steps to make sure that someone would be obligated to do so.
The subletter for August failed to show. My roommate started emailing me asking about reimbursement on July 25, and I informed her on August 10 that there was nobody in her room. She is now threatening to take me to small claims court because my "withholding of information prevented her from finding a subletter and caused her material damage". However, from my perspective, she couldn't be bothered to be involved enough to know who was in her room and sign a contract. She's threatening to take me to small claims court, and my question is - does she have an argument that would hold up in court? My initial thought is that she does not, but I'd like to be sure before deciding how to proceed.
I've since moved out of state, but as I come back to MA often I would like to be prepared.

If you prepaid rent to your landlord, then your landlord presumably will apply that rent to the last month of your lease. Your landlord won't care if you're squabbling with your roommate. If you believe your roommate owes you money because your pre-paid rent was applied as intended, you will need to work that out with your roommate, perhaps through small claims court.

If you had an agreement with your roommate that they would satisfy their share of the rent through a sublease, then you changed your mind and prevented that from happening, then your roommate has a plausible claim that you should pay the full rent for what would have been the period of subtenancy. The rest depends on the detailed facts.