Well, the way to deal with this now is to let it go (hard hard hard). You talked to them. they want to believe he ripped the check up. End of story. You tried.
Just go on and love them for who they are and hope that when they see the bank statement they will know you were correct. It is theie choice.
Please do not look at my statemnet as any sort of judgement cuz man I can sure understand. My Dad is 85 and Mom is 80.