if it is your opinion that this bet would allow excessive risks, then it is your responsibility as the owner/operator of MoneyPot to disallow such bets from being placed. As the owner/operator of MoneyPot, it is your responsibility to ensure that your bankroll investors are not exposed to excessive risk.
The problem is he (and probably all the investors) didn't know. In fact, even dooglus's first post in this thread said "I don't understand why this bet was allowed to go ahead...I never looked in detail at how the kelly criterion works when there are multiple mutually exclusive outcomes like in the case of plinko bets" and RHaver said "I'm quite surprised it would allow 0.4 BTC bet -- that seems a bit much to me". They are only aware of what Kelly formula suggested in this case now, after seeing that bet happened and re-visiting the math behind it.