Wow, is it that big a deal? She's just towing it to you place, not actually using to go camping, right?
If she has a history of stuffing things up or breaking things then maybe tell her where to go, but if not, its just towing a trailer! Its not like a mate is borrowing ya wife for the night, it won't be any different if it isn't you doing the towing, the trailer won't self destruct because its not being towed by you!!!!!!
Can she tow the boat instead?