But that's what the whole case turns on. They DID have direct knowledge of his guilt because the proffer was that Westerfield would tell the police where the body was in exchange for a plea of guilty to a lesser charge.
Indulge me for a moment, and I'll explain what I'm thinking. Criminal defense attorneys almost never ask their client whether he is guilty. They don't want to know for exactly the reasons we are discussing: knowledge of their client's guilt restricts their options in presenting a defense. It is possible that Westerfield admitted his guilt, but the only people who would know that are himself and his attorneys, who apparently have not commented publicly. Neither O'Reilly nor the Tribune reporter can know directly; they are inferring the fact that the attorneys had knowledge from the fact of the plea offer.
From what I know, defense plea offers are normally broached in the most non-committal way possible, and I would envision it happening something like this: "If my client takes you to the body, can we get life w/o parole?" Obviously, Feldman had to accept at least the possibility that his client was guilty; why else would he be on trial?
If the prosecutor accepted the possible plea deal, it would have to be approved by the client. Feldman probably would have said something like, "It doesn't look real good for you, but the prosecutor has said that we can get you life if you produce the body." It would be then that the attorneys would be, in effect, asking the client if he's guilty. But we know that it never got to that point because the prosecutor found out the body was found.
If they had taken the deal to Westerfield, and he had maintained his innocence (that he couldn't tell them where the body was), all that Feldman would have done is inform the prosecutor that his client maintains his innocence, and the trial continues, no harm, no foul.
Someone pointed out on another thread that this isn't necessarily so. Feldman could have simply told the prosecutors "offer a deal to accept a guilty plea with no death penalty in exchange for locating the body, and I'll present it to Westerfield." What Feldman didn't say to prosecutors was "and I'll advise him to laugh it off." Under this scenario, Feldman was just trying to feel out whether the detectives already had a body. Time line mattered to Feldman's preparation of arguments.