The Montessori Daycare down the street from me will not accept children that are not potty trained. Perhaps your friend led them to believe her son was potty trained when in actuality, or perhaps because it was a different environment, your friend's son would have accidents.
Did she go over her contract?
Besides..why is your friend so interested in legal advice? Is she planning to sue them, or fight her son's spot in a daycare that does not want him there..as a mom, I would not want to bring my child to a place where he wasn't wanted. Just sayin.