Interesting one there Pilot DAR, which country did that occour in?
As far as I was aware, if someone signed as PIC, they were responsible whatever.
So lets say if it turned out not only that pilot A didn't have a medical, but also the ARC was out of date, he would still be responsible by signing in as PIC. Are you saying pilot B could somehow be held responsible for the flight just by having a valid licence and being there for the ride? Bearing in mind in this example no accident has actually occoured.