Her lawyer said there was no plea bargain. Who else would you expect to have heard the conversation?
Of course he'd say it was a "misunderstanding". What else could he have said? His job was to inform his client of her rights, to give legal advice as needed, and that would surely include informing her of potential sentences in various scenarios, wouldn't it? Pretty useless lawyer if he didn't.
As her lawyer, it makes sense that he was able to view the so-called evidence in order to advise his client on the "best" course of action.
When I first saw Eddie barking at the side of the car, my first reaction was that I found the footage quite chilling, as I'm sure many others did. The lawyer may well have done as well.
If that's the case, the PJ may have hinted that they were considering a murder charge in the light of the "evidence" (whether they really were, or it was a bluff or a genuine total misunderstanding). Alternatively, he may also have wondered himself whether the dog "evidence" indicated a far worse scenario than he'd originally thought.
Even if he thought she was innocent, he may have realised that the PJ might be out to get a charge anyway, and so advised her to consider pleading guilty to an accident, with a lesser charge than if they were to go for murder. Gerry and the kids could have gone home, etc.
However, if that was the thinking, I find it somewhat naive, particularly in the light of the other missing child case. If she'd falsely pleaded guilty to a lesser charge, there's no way in hell that Gerry would have been able to go home.
And the inevitable next step is "where's the body?"