depends on the guy. obviously he could do what the above guy said.
but there could be a few reasons for it. there could be two people just simply using the same login.
i was in a league with two brothers who used just one yahoo ID, so the same email was used by both, but believe it or not, in the 3 years i played with them, i never ran into a single issue.
they did trade a fair bit with each other actually too, but i suppose if you live 2gether, it is a lot easier to find common trades, and if you have two teams, it would be a lot easier to make trades for the needs of each team.
although i would monitor it very closely. however, in this case since he's inactive, it doesnt seem like it will matter.