How far you get depends entirely on the honesty of the JW you're dealing with. You can score some big points, though, by trapping him into an admission by doing the following:
First give him the quoted material, and have him read it out loud and comment on what it means. Make sure you draw him out on the specific misquoted material, and get him to say very clearly what the material means. The point is to have him state, for the record, what the material means, in his own words.
Next, have him read the Watchtower material. Ask him about the quoted stuff. Ask if the impression he gets from it matches what he just agreed to when he read the original. It's critical to get him to see a discrepancy. Most likely he'll hem and haw and do everything he can to avoid admitting that the material was misrepresented.
If he absolutely refuses to admit the misrepresentation, then you can handle it as you see fit, including telling him that you refuse to go along with such blatant intellectual dishonesty. If your wife is looking on, surely she'll note this with chagrin.
If he admits to the misrepresentation, he most likely will try to play it down. He might say that it's really no big deal in view of the many other fine things in the book. That will give you another opening: "But this isn't the only one! I've found lots of other misrepresentations." Again you'll have to play this by ear.
The overall goal is to try to get observers to see that misrepresentation of sources is not just an occasional occurence in Watchtower literature, but is fundamental to it. If an observer sees this, it's probably all over for his relationship with the Society. If he sees it and still goes along, well, that's his stupid choice.
Some years ago a German man who had briefly "studied" with the JWs wrote me about his efforts to get a clear answer from the German Branch about a gross misrepresentation in the 1985 Creation book (on page 143, of zoologist Richard Lewontin; see http://www.geocities.com/osarsif/ce01.htm). After close to a year's worth of correspondence, and even correspondence between the German and Brooklyn Branches (Brooklyn simply claimed, without explanation, that there was no misrepresentation), the Branch admitted that there might be a problem, but it was no big deal. In 1997 I confronted the author of the book, one Harry Peloyan (editor-in-chief of Awake! ) about the overall tone of misrepresentation in the book. He refused to admit anything at first, but eventually conceded that there might be a few tiny problems. He claimed that such small problems meant nothing because the book was such a fine pointer to Jehovah and the Kingdom. So that's about what you can expect from rank & file JWs, too.
AlanF