Head and Shoulder (insert dandruff joke here)

[Rule11]

Worked a game last week and a situation came up with my partner that was perplexing to me.

During a fast break B1 obtains LGP and stands like a statue while A1 drives to the basket. A1 realizes that B1 is in good position and makes an athletic move to twist away from any torso to torso contact and brushes B1's shoulder as he scores the lay up. Of course B1 goes down like he was shot and B1's coach is yelling for a PC foul...I think my partner does a great job with a no call and we play on.
During a break in the action B1 coach nicely asks why no PC call and my partner says that there was really no contact BUT if there was contact after A1 had gotten his head and shoulders past the defender a foul would have been called on the defender. I am a little surprised to hear this as B1 literally did not move but I figure we will just talk about it after the game...no big deal.
After the game I ask my partner what he meant and he said that if A1 had gotten his head and shoulders past a stationary defender (Who had LGP) and contact occurred with the hip/leg etc it would be a foul on B1 because A1 had gotten their head and shoulder past B1 and therefore B1 no longer had LGP.
To back up his point he says that we discontinue the 5 second count when a dribbler gets his head and shoulder past the defender and this is really the same situation.

Reading Rule 4 Section 7 Art 2 in the NFHS rule book. I see that it says that if there is torso to torso contact the onus is on the offense which is clear as can be. It also says that if the defender has gained LGP the offensive player must get his head and shoulders past the defenders torso.
I just don't see anything in the rules where it states a foul will be called on the defender in the situation noted above- nor in my opinion would it make sense for a foul to be called on a defender in that situation.

Just want to make sure I understand the rule correctly.