Definitely look into the laws, MA I presume would be employee-friendly.
But, while you would have a case, and probably win any suit if fired over this, or the age aspect, you have to be prepared that might only pay off with a fight, and a lawyer (though perhaps the state labor board would manage that). Still, it's not nothing and to say "oh she'll win a nice lawsuit" - yeah maybe, after many months or longer, and maybe not that nice since ultimately damages are paychecks. A big payout from a labor lawsuit isn't ironclad.
Even if she wins, it could be something small like a year's pay and then that's that. She's been there less than a year, so they could quite well be smart enough to find cause or "new person, not working out" and fight. It's a risk she'd be taking on. Not any of us, just her.
OP you have to ask yourself if you have it in you to fight, and can afford some risk in case it comes to that.
Or, if you make $X for 35 hours work, would it still be decent pay if it was still just $X for 40 hours?
Do you like the work, your boss, your colleagues, your commute?
Do you hope to do this for 2 more years and then end nicely in retirement?
If I were in final 2 years of coasting, unlikely to find something that paid as well that I liked, I might just suck it up. And then find ways to get those hours back. A long lunch here and there, a late start some mornings, etc.
Oh as as far as "but there's just so much work I need 40 hours to get it done" - respectfully, when I was still working I learned that it didn't matter if I worked 40 or 50 or 60 hours, there was never enough time, there was always more that needed to be done. But I was only getting paid 40 (salary) so I stopped with the 60 hour weeks and dialed it back in those final years to a comfortable 40-45.