Well, it kinda depends on what the lease agreement says. Definitely get a copy of that agreement. Generally, when a person leases a horse, THEY (the leasee) keep, house, and pay for any expenses the mare incures. BUT, as the owner, you have final say about the care etc. of the mare. Generally, but not always, the leasee keeps and foals out the baby. A lot depends on the agreement. I've known some people who own mares to make it part of the agreement that the mare foals with them.
Being as its your boyfriends mom, I would tread lightly and try to come to an agreement. Not saying to have your mare in an unsafe environment, but try to get to a common ground. For example, maybe you could keep your mare where you want, but you be sure to call her when the mare foals so she can be a part of it. Or, she can keep the mare, and call you when she foals so that you know the foaling is going well for both dam and baby.
Is there a disagreement going on? Or is this a general question?