Usually it is dangerous to let such people write application level code, but according to the reports in the comments, the guy might be one of the few exceptions, who gets the low level right, but also created flexible extensible structure of code in other places, without it becoming obscure on the application level.

Usually when I see some mathematical code that's not explained at all and has obvious flaws in terms of simple improvements that could be made to improve readability or even something as simple as some comments, I just keep thinking: "Please don't let this person touch application level code!"