Email the league reminding them to submit valid line-ups or face expulsion from the league. You can't change his line-up this week unless you have something written in the league rules about a commish override of the line-up. If he continues to be a awol owner, out he goes.
Agree with the above and have a question I'm surprised no one has asked. Have you been viewing hist eam all along? Has he done this before? If so--did you update his line up that week? If not--no way do you do it now... you have to be CONSISTENT... if you manage his team for him one week... you have to do it EVERY week--no matter who he's playing... that's for the rest of the season. It stinks when people AWOL on their teams...
but just leave it and everyone gets the same benefit--a free W. Manage his team for him and you open it up to biases (ie. you beat him when playing him but he beats other teams when they play him--league mates will say you were 'tanking' against yourself and trying against them...). I'd let it be and if he's gone AWOL boot him from the league. If its a one week thing... as others said rag him for it.