Aw that is sad. She would probably do well with a really high fat and high nutrient diet to keep her weight good and help her overall health. Is the foal eating yet? Purina has a great feed called Omolene 300, it's a growth feed recommended for pregnant mares and foals. Wean her slowly onto that and a high quality hay and you should be in great shape..
Purina Horse Feeds - OMOLENE