Pardon me for butting in Greg. I assume that the only evidence of this conversation is Dureau's memory. Since many years passed between the time he allegedly talked to Shaw and spoke about it to Mellen, he could be misremembering details. Or, he could be mixing his memory of an actual conversation with things that he has read in the interim.
I have no way to prove it, but I believe that late-occurring memories by individuals like Dureau (and many many others) are likely influenced by what they have read about the case. Take the case of John Armstrong and his "witnesses." These people were told that they witnessed something significant, something that changed history. That is a powerful motive to "remember" something new.