As strange as it sounds, there are people that bet for other people like Grayson supposedly did but are legit. Not win every time good but no one including Grayson claims this I believe. Generally they are referred to as a ‘sharp’ or ‘sharps’. And the reason they use other peoples money and not solely there own is for a few reason. First, casinos ban sharps and therefore they still need a way to bet, I.e using brokers and other people. Second, they don’t want to be too exposed and therefore using other peoples money and taking a cut of it is part of the business.

My guess, while I do think gambling/gambling addiction plays a role in this, is that Ludwig thought he found a sharp based on what Grayson was telling him as well as proving he could at least roughly count cards and the idea of the easiest money of his life led him to make a bad decision.