Bottom Line- The boyfriend is being dishonest.
I cannot believe for an instant that someone would not notice being hung up on something. On or off a trailer- If you are not able to move, increased throttle is NOT the answer. Doesn't matter if it is hard and fast or slow and gentle- even someone with little experience should be able to figure it out.
The amount of force needed to bend that ROPS as it is- had to be great. Whatever happened, the event HAD to have been noticed. In his defense, if it was someone else/not witnessed by him, I might understand ONLY if he was up front about it. Very wrong for the owner to have to ask for the answer you already know you have to provide.
Hard thing though- asking to pay for repairs in retrospect. As suggested, his homeowners policy may help....... so many things in the fine print... Even harder....splitting a family over this. Almost a no win situation at this point, and truly the exact cause is moot. Something happened while he was responsible for it. If he helps pay great, if not- your choice as how to pursue it further.
Sorry that you are in this position. Good luck- and I hope you are able to establish trust again.