That's what I love about email!
I would hope for the child's sake both parents are contacted so that every avenue to have someone assist this child is explored beginning w/ the parents. However, if the school has court documents or other direction on thsi matter I suppose you'd have to side w/ the courts. I think I'd talk to the principal on this one.
Personally, if I could get away w/ it I'd call dad AND mom and tell them each the same thing and let them both know I contacted both of them. I'd hope everyone could keep in mind you are trying to help the CHILD here not stir up their divorce.