He will try to find some minute detail that is wrong, sue on that, and if he wins, he and the old media will all talk about how the Swifties were sued and lost and thus discredited. He doesn't have to prove the whole book is false, just one piece.
That's very simplistic. There are a couple of errors concerning the 13 MAR incident, such as mention of a PCF-35 (which Corsi based on Brinkley's book, I guess). It doesn't matter. And the mistake that Screamin' Larry O'Donnell harped on was O'Neill suggesting that the last line of the report indicated who sent it, rather than who received the message in a separately typed line. But there seems good reason to believe the assertion that Kerry was its source based on the time it was sent, and just who would have been around at that time of night on the US Coast Guard Spencer, which has been identified by the sender's routing indicator.
I was, in fact, one of those principally involved in the threads at the Swiftie site where this mistake was uncovered, based on historically later documentation of what these messages contained, how they were blocked, etc. The other, 'NavyChief', who had discovered this timeline and series of likelihoods later on, and myself and others were quick to point out that while the former was a mistake in reading the dispatches, the latter essentially makes a very tight argument that the original assertion was true. In fact, this was precisely what O'Neill was trying to say as Larry performed his very bad Sam Kinnison impression on national tv. A) none of the other officers that day now claim to have written it - leaving only Droz, who was KIA shortly afterward. B) it's an account that favors Kerry and seems to ignore the essential events when he went absent down river. And C) only Pees and Kerry would still have been aboard when the message sent.
The only other complaint Kerry might have is what turned out to be the main complaint with Kerry, that day, just noted above. It's not whether they received fire. That's logically ludicrous if one examines the action reports. The main complaint with Kerry was that he fled in the face of what he believed was hostile enemy fire. Thus, there appeared to be a pattern by his Sandusky and Thorsen, et al, to constantly brag of Kerry's bravery, of facing fire, even of being eager to join the Navy because he was 'gung-ho' for firefights, all of which was contradicted by other accounts even favorable to Kerry; and all an obvious cover for how Kerry behaved, generally, and specifically that day. In fact, if people remember the idea that Kerry fled in the face of what he believed was enemy fire, that one idea, alone, might have cost him the election with many voters.