Assuming, as you requested, he was moving toward B's goal line, then
a) if somebody saw him entering the game, they should have so signaled and, since the snap was imminent, the U should have stepped up and prevented the snap. Easier said than done, but, an astute R (and U) would have recognized that A had only 10 players, and should have been checking for a possible late substitution, or even T/O request by the HC. If either saw the late sub, they should have attempted to prevent the snap; even after the ball managed to get snapped, they should have shut the play down and played a "do over,' giving B due process to 'match up.'
b) if the crew fails to follow proper procedure for the late sub, at least they should have recognized the illegal motion (moving forward at the snap) and/or the illegal formation (more than four in the backfield at the snap.
Someone suggested a holding was missed. No. Yes, the blocker had a handful of jersey, but he stayed in front of the defender, and continued to drive him in the same direction. There was no pull-down or twist. The defender did not demonstrate a significant attempt to get away from the blocker. Simply not big enough.