It depends on the broody. Some do fine getting up to eat once or twice a day. Some actually die from being broody. Almost all lose some weight from not easting as often/as much. If you are able, I'd take her off the nest and encourage her to eat/drink/peck and scratch at least twice a day; I've done this many times. They usually fluff up at me but they do get some food in before returning to the nest. Sometimes a week or two of this will break them; if not, at least they're eating. I've tried some of the more extreme measures of breaking a broody and have decided if they're that determined, it's just cruel and not going to work anyway. So I just make sure they eat, or give them eggs. I've had no luck hand feeding them on the nest, or leaving food/water in reach from the nest. IME, you have to get them on their feet --- and a bit irritated with you.
Really, the best way by far of breaking a broody is letting her hatch, or giving her new chicks. Even where you are, I'd probably let her hatch a few, especially if you have fertile eggs at this point. They do remarkably well even in this cold, and you don't usually have to do a thing, they take care of the babies. I don't even separate the broody from the flock.