When I played darts to quite a high standard I would never play for a bet more than the price of a drink. It was a social activity for me, and putting large sums on it took that away. If I were you I'd give him two to one against any fly "he has tied". Thats his challenge to tie one better. Yes I know he doesn't tie, use it to get him to. He will understand much more about his fishing if he learns to tie. If he will not do it then use some other forfeit.
Cheers,
A.