[Jim S]

OK PiggSkin, I'll relent. There is a foul there. It's not IS because he is still a player unless you determine that he left the field to gain an advantage. It's not IP same reason. It is (drrrrrrrrrrrrrrrr(drum roll)) a numbering violation. 5 yard foul at the snap. You see the numbering violation section is the only place in the rulebook where it defines where B can be. It says B players must be on their side of the LOS. When discussing position it is assumed but not stated that positions are on the field of play.
We're not going to let him get away with coming on after the play starts.
This BTW is the same foul if a B player lined up in the A backfield and A snapped the ball. Numbering violation. 7.2.5 Have won a lot of bets with this rule.