What matters is the betsize in relation to the total amount of €400 that is bound by WR. Exactly because a player will bust out very fast quite often is what makes it +EV for the player, because busting out means not wagering. The casino gives €200 away and the only way they can make it back is by the player wagering. What matters is not the individual results for a single player but the combined results of a lot of players, which will give the casino something like 5% back on the total wagered, assuming 95% RTP on slots. It's quite counterintuitive, but if someone wagers only €400 the casino only makes €20 back of the initial €200 they gave out as a bonus, just that it's not that particular player getting the money, it's one of the others playing the same way that will hit a big win.
There are some old threads explaining it more in detail with examples but it doesn't matter much for this case since the player broke no rules and should get paid. Just did seem a lot of people weren't aware that this was quite clearly advantage play and it will be the regular players that will end up having to pay for it.
https://www.casinomeister.com/forums/threads/new-rival-casinos-lost-the-bonus-plot.33486/ is one of these threads.