@Joneszzz Yeah I see now you're right about that, the arm isn't really broken until after the bull.
There is one thing, tho - It's the arm that Law just shock willie'd. BM did comment specifically on her ribs being cracked from Law's internal shockwave, and that it was his technique that was dangerous (or it's kinda inferred, she says "shockwave from inside, wow!", "cracked some ribs too", "Law's moves are dangerous" in that order - it makes the most sense that all three are referencing Law's attack, bc the ribs comment is sandwiched between the other two clearly Law-related statements). So, her arm was already fractured by the shock willie, like some of her other bones - and it was right at the center of the attack. Because of that, blocking Kid's attack broke the arm.
Which is in line with how the fight went, Law doing most of the damage and BM mostly talking about him when it comes to her being hurt, and Kid she mostly has been noting about him still getting up and being alive, while he provides follow up attacks to Law to keep BM off her feet and take advantage of how Law's attacks weaken her to let Kid do actual damage. Law has pulled some insane bullshit out of his ass this fight, his attacks are doing an insane amount of damage even with BM's fruit and durability. It's kinda warping the balancing of the whole series (like all the yonko fights on Onigashima have been), but in some ways the strategy of this fight has made sense.