No foul for BIB.
9-3-5b. A player shall not block an opponent in the back except:
b. Using hands or arms to contact an opponent about he waist to [...] catch or recover a loose ball which he may legally touch or possess.
Now, you could make an argument about illegal use of hands/holding if B wasn't making any attempt to go toward the loose ball and was only trying to hold A back, but that would have to be *really* obvious/blatant for a hold in that scenario.