Karen A good place to start if you want to learn about sub metering and re billing for a landlord supplied utility is start with your states public utility commission asking for the rules for sub metered billing of landlord supplied utilities, 2nd place to search is your states landlord tenant laws, 3rd is to learn from your city if they have any ordinances regulating residential sub metered utility services,
https://www.mass.gov/info-details/massachusetts-law-about-submetering-water-for-tenants has sections that address this. ( lastly you could change the thermostat in her unit to a conservation model so it cannot be set to more than 70-72 Keep in mind that this way of doing it doesn't stop the heat from coming on while windows are open ( old well known round thermostat manufacturer makes them and Ive used them in the past) OR even consider having a heating tech change the wiring so your units thermostat controls both units and be ware of state set minimums for heating degrees and defined heating season when utilities are included. Keep in mind it is up to you to verify that the information in these links is still valid
https://www.malawforum.com/content/electric-and-heat-submetering read
update on separate utility bills in Ma as well as the others above it. Lastly to make sure you should consult a real estate attorney who has a lot of landlord tenant background. ( feel free to talk to a local electrician to determine if your rentals electric service is large enough to handle electric heat and to find out what it would cost to install energy efficient baseboard electric heat. There are baseboard units that use less energy than cheaper standard units) Lastly if this is a hot water or boiler system with each unit having its own thermostat then it may well be affordable to have a second boiler installed to serve that unit even if its a all electric model since the piping is already separate.