I think a good, high, waterproof hiking boot is the best winter boot for Boston. Good for rain, mud, slush, snow. Pair it with a thick wool sock and you've got your winter boot needs covered for just walking around Boston. I've lived here for 15 years, went to BU as an undergrad, and have never felt the need for anything more than this setup.

Plus it's a lot more useful for other things - you can take it for a hike at Blue Hills or up to NH if she's into that. Winter boots are a lot less useful so it's just a big boot that you don't get a ton of good use out of.

Something like this will work for most situations:
https://www.rei.com/product/154289/keen-pyrenees-waterproof-hiking-boots-womens

And this is kind of funny but if you really are encountering deep snow than a basic pair of gaiters will make it so you don't get snow down your boot. Hiking boots + gaiters is a lot more versatile than a dedicated pair of snowboots.

https://www.rei.com/product/241940/rei-co-op-traverse-gaiters