"When I'm talking about 'truth,' as opposed to 'opinion,' I am talking about whether or not Joe made those comments, not whether or not John's wife really is a whore."
Ok...I see now. The alleged defamatory statement is Bob's statement to John that "Joe called your wife a whore." In that case, then Joe will have the burden of proving that Bob made the statement to John. As noted in my prior response, who has the burden of proving the truth or falsity of the statement (i.e., whether or not Joe actually made the statement) depends on the applicable state law.
"If John really did make those comments, then Bob's statements are not defamatory, due to Bob's statements (regarding Joe's statements) being true."
Now you've raised -- probably unintentionally -- a different issue. Whether or not a statement is defamatory is not dependent on whether or not it is true. A true statement can be defamatory (i.e., "Dave has a loathesome disease" if, in fact, Dave does have such a disease). In your hypothetical, the statement at issue "Joe called your [John's] wife a whore" is arguably not defamatory because doesn't implicate Joe's character or reputation. It's no different in kind than a statement that "Joe ordered a BLT for lunch" or "Joe went to H**** and cheered as the Habs took the first game of the series from the Caps." In theory at least, John might fire Joe for going to H**** if he thinks H*** is a sexist establishment or for cheering for the Canadiens if John happens to be a die-hard Capitals fan. If, however, instead of going to H***** and cheering for the Habs, Joe watched the game at home and cried himself to sleep after the Habs stole the game in OT, the statement would be false, but it wouldn't be defamatory. And, if John fired Joe based on Bob's false statement, Joe would not have a viable defamation claim.
"If Bob completely lied about what Joe said in an attempt to get Joe fired, then the statements are defamatory, because, for example, Joe didn't even know that John was married."
I would take the position that, while Joe might have some other sort of claim against Bob, he doesn't have a defamation claim.
[This post has been edited to remove personal or identifying information. -Moderator]