If they agree on a time when the bets will be placed, then the documentation will have the timestamp, and it won't be possible to cheat. Plus you can easily produce the entire history of bets under a name, and so you have a record of all bets. The person placing the bets can't hide a win, and simply do another bet at a later time.
The person placing the bets can simply agree to only place N bets per week, and never 'randomize' the seeds. Then each week he publishes his N betids, and all participants can look up the rolls on the site and see that the seed hash is the same as it should be, and that the 'nonce' values are consecutive.
If there are any gaps in the nonce sequence then he's probably hiding a big win from you.