As a parent I think he has every right to know more about the trip. As I read it, his then 15 year old daughter is going to the USA to visit theme parks with about 15 other kids, no details of who's in charge, supervision, itinerary, accommodation, travel within the USA etc.
Before signing a passport application he definitely needs further details. Who is organising the trip? Is there adult supervision? Who will be in charge of these teenagers? !5 year olds may think they are grown up but a group of teenagers alone in a foreign country is a recipe for trouble. If its a school trip or similar it shouldn't be that hard to provide him with details - those that are known now and further details as the trip gets organised.
If I were him, I'd make it clear to his daughter and her mother that while he is not outright opposed to her making the trip he will not sign the application until full details of the trip are given to him. And he needs to discuss this with her mother like adults and not through his daughter - at 15 she's not going to understand or appreciate his concerns and is likely to get highly emotional if she thinks he's preventing he from going.