Step #1 would be talking to her. Have you asked her about the situation? You can calmly say, "The bills seem a lot higher than in the past. Are you using the thermostat? Could you lower it during the day, when you are at work and your child is in school?" She may not have realized that this was an issue, or you may learn why she is doing this (a window that won't close fully, for example, letting in cold air.).
Another step may be checking to see if your furnace is working properly.
If the lease says that heat is included in the rent, those are really your options, until the lease expires. .
Dear Staten Island Landlord:
The legal minimum heat during daytime hours and evening hours during the heat season is 68 degrees. No law prohibits heating above and beyond the minimum.
If the lease includes heat as a part of rent there seems that the tenant is within her right to heat to her comfort level even if above the minimum you would need to provide.
Of course you may choose to inspect the apartment for a functioning thermostat and loss of heat due to leaky windows and drafts. You are allowed to weather proof when needed and replace a defective thermostat.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.
Can you relocate the thermostat and/or install one that you can program from a remote location?
I may be guessing or not licensed in your state. No atty/client relationship exists.