Well, it's definitely section 7.
But does he have anything in his agreement about being consulted first? He could argue that she should have discussed this with him so that they could decide it it was possible, not just have her send him a bill with no input. But what does 'signed up' mean? Are they committed, or just expressing interest? When is the trip? When is the deposit due? Do the students do any fundraising to offset the cost? Can he suggest that the child help earn some money for the trip?
But, what are the best interests of the child? A school trip overseas is a great opportunity. If the couple was still happily married, would they make it happen for their child?