depends,
was there any contact? if not - nothing. However, B1 is not required to move just because A1 does. So if B1 had gained LGP and was in LPG when contacted by A1 whether he was going to basket or making a concerted effort to avoid contact, it is a PC on A1.
Would you tell the coach of A1 that you didn't call a block on B1 when they realized they didn't have LGP and were making a concerted effort to get out of A1's way when contact occurred and A1 lost the ball?
