The question seems to be discussed in The Philosophical Quarterly and the author seems to agree with you guys. However, as the author is in fact opposing a previous stance by a different author, I am still not convinced.
Also, legally, it seems that if the precise action that caused a crime cannot be determined, all the involved parties are tried for the severest crime that they could be tried for. This seems to be more acceptable to me. Refer to a google of the words 'Mises' 'Ridiculous' and 'scenarios'.
I am more confused than I was to start with, but it seems like a lot of people agree with you guys. I guess I need to analyse this further philosophically, but legislatively, your points of view would win... Unless B had a kickass attorney
Cheers!
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Vig