Normally they get off the nest once a day, and yes, normally they lose weight. I take my broodies off the nest whenever I'm in the coop, whether they are hatching eggs or not, as they get a little more exercise and food that way. I prod or pester them til they get moving around if needed. Right now I have one who will just plop down by the nest, so I keep after her, and another who runs outdoors, clucking madly and running around (yea!) I wouldn't put the water right by her -- I'd pick her up and prod her to get moving. She will find her own water, maybe even forage or dust bathe a little.
Or you may want to try to break her broodiness. Put her in a cage with no nesting material and with air flow below her for a few days, or put her in an area such as a garage with a concrete floor, again with no nesting material. There are several ways, none of which seem foolproof. I don't really try to break mine, I just make them move around.
If you can get hold of a chick or two, the other thing you can do is slip day old chicks under her just after dark. By morning she should think she hatched them, and will raise them and protect them from the rest of the flock.