Depends. If he can now spend all his time doing what he likes to do, why he would want an additional burden of managing all those money? And even if sometimes in the future he will be in a serious need of money, many math institutions or universities will be more than happy to employ him. So even without accepting that money, he can still do what he loves to do all his time. Happy man :)
He could choose a way of managing the money that was optimized for simplicity. For example putting it all in a one low risk low return income generating instrument. It's hard for me to believe that the stress involved in picking up a decent cheque every month for the rest of his life wouldn't be outweighed by the benefits.
To me his actions are those of a "difficult" person. Very hard for a more relaxed (dare I say "normal") person to understand and rationalize. So I wouldn't bother. As others say, maybe that's part of what makes him different and special.
By decent I mean "worthwhile" or "significant". I specifically didn't say "enough to replace the income of a comfortably placed person in a first world country". 2% pa yields what, $1600 per month ? Don't you think that would be worthwhile to someone living in poverty in Russia ?
