Stop asking people to do this, if they felt it was in their blueprint or plans it would be included.
It just annoying now. Just dont play if you dont think he is fair. Additionally, He may not even know what that is. You could say something like, "It would be easier to garner trust in this specific community (which is small and insignificant in gambling terms) by providing a service to prove the fairness of your bets...just a suggestion" You make it sound like it is a requirement or something even though like no gambling site ever has ever brought that up or offered such a service to its patrons.
Shut up about provably fair for gods sakes
it will reveal itself through analytics if you really care so much.