Sometimes friendships are more important than the fantasy league. I commish a similar league and, unfortunately, I sometimes have to make a phone call and get someone to set their lineup. It takes me a few minutes, but we avoid these issues, and I can give them hell for having such an awful team. I know everybody is quick to point out the accountability piece, but sometimes we have friends that aren't into things as much as we are, but having them there is more important.
For your case, I think you have to leave it as is. If you've never stepped in before to set precedent, you need to deal for this week and address it in the offseason with a rule.