It's a chemical reaction between the strings and his fingers. I have this happen to me, regardless of string type or brand. I've found it's much more noticeable with older strings than newer ones, but it happens on all of them.
It "could" be the strings themselves too (my first set of Circle Ks had my fingers extremely black after playing every night), but that's more of an exception. I'd bank it on his fingers over everything else.
Best advice is to wash your hands before playing and wipe down the strings afterwards.