What on earth does Massachusetts law have to do with it?
If the car is delivered in NY, regardless of where the buyer actually legally resides, then the NY Sales Tax is due. You might be able to convince a New York dealer to deliver the car in NH and if she has establisehd residency there it might escape the tax. However, she probably would do better buying in NH. There are a lot of dealers especially close to the MA border.
If she buys a car from a dealership in NY then they will issue her an in-transit temp registration.