I'm tired of fighting with online bookmakers, which in 99% of cases are scamers, ....
That's not true, if you look around in this section, I'm sure you can find a lot of reputable gambling sites. And how much are we talking here by the way? Are betting around $1000, $10,000 or $100,000? For sure some sites can handle that huge bet, I don't want to recommend a specific gambling site but just look around, the more their ANN thread is active, that means they have a lot of gamblers supporting them.
Stake and Sportsbet.io can handle huge amount of bet but I'm sure the bet size is not his issue since he is finding partner to make a bet for him. It's either he will offer a malicious partnership proposal or he leaves in a country which sportsbetting is not allowed or with some restrictions.