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

We are 4 of us (not friends) in a single house/apartment unit. The Gas bill is in my name. Now we have a flat mate who is moving out probably (she refuses to disclose when she will move out and who will come in place of her) by the end of this month (January 2014) and she is subletting her room to another person. Now she says she will only pay for as many number of days that she will stay at this house and nothing for the remaining days of the monthly bill cycle. Our gas bills are something like $300-400 a month, so I can't forego the remaining amount if she abruptly leaves in the middle of the cycle. She says I should collect the bill for the remaining days of the cycle from the new sub-tenant and that she will pay me bill according to previous month's bill. How can she do that? What if the bill for Jan is more than Dec? And who do I calculate how much usage per day occurred? Isn't she liable to pay for the Jan bill cycle if she leaves in the middle of the billing cycle? What if the new subtenant refuses to pay in this way, and what if there is a gap between this flatmate moving out and the new sub-tenant coming in? Is it possible to tell the gas company and not pay for that flatmate's unpaid bills? Please suggest a course of action. She has saying the same for electricity bills also which are high too. It is written in the lease that all tenants have to equally pay the utilities bills, irrespective of whether they are at home or not.

If I fill a claim in small claims court, what are the chances I win? I can show all the required proofs. Please reply as soon as possible.