Is this her first foal? The chances are higher of her slipping because of her age and increase if this is her first foal. (sorry) But, that is not to say that she WILL slip. Keep an eye on her, make sure she has gentle exercise such as walking, be sure to feed her quality feed and hay. Be sure her weight is good, but don't let her get fat. Being fat is hard on the body and increases the chance of slipping.
I would be carefully optimistic as to her chances of keeping the foal. We have had several mares deliver in their 20's with no problems. Good luck and keep us posted.