I can't stand it when people will give kids things after another adult has already told them no.
It doesn't really matter if you agree or not--unless the first person has been totally unreasonable and is denying the child something they absolutely need, you should not undermine another adult. You just teach the child not to respect them and that isn't right. This is especially a problem when the kid learns to play one parent against another and they parent will let them, in an attempt to one-up the other.