The baby is ready to be weaned. If anything, the baby is putting a huge toll on her mom. When they get over a certain age, they need to be weaned, especially if the mom isn't totally healthy. How much acreage are they on? Is there grass? If anything the horse should be getting grain, even if they don't get hay, especially if she has a baby. Baby should already be getting grain, too, to supplement itself. You need to call animal control, they will assess the situation and make their own judgements. There is a possibility they won't be taken away, but the girl does not sound very nice and it would be a great benefit to the horse if she was taken away. What do the girls parents say? Do they do anything?