Umm she's pregnant and eating the grass from last summer under the snow? That should be a red flag right there. Is that all she has to eat out in the pasture, or do they get hay as well? If she is in foal, I would make sure to have a vet check her, and perhaps re-evaluate her diet as well. Horses need to have rough forage like hay in the winter to help them maintain body heat as well as weight. If she is in foal, she is going to need double to nutrition, especially in the winter. If you are just looking after her, I would contact the owners right away.