She said he told her he'd paid a hitman to do it.
So over a period of-Sorry Bridget, hi-months he gets her used to the idea that he could kill his parents by employing various methods. He quite openly shares these macabre fantasies with her,...... so she says. The day of the tragedy he calls her early with something along the lines of "Tonights the night" or "It's now or never" and later calls her with something like "Something's wrong at the farm"....... or so she says, all of which, on its' own may have been enough to convict. But let's suppose, having vented her spleen to revenge herself on his affair, she doesn't feel quite so confident about the thought of him receiving a life sentence because of her, and because she may well, despite being furious at his betrayal, still have been in love with him, so she decided to bring in the hitman to remove Jeremy's hand from the gun.
Nobody can verify what was said in these phone conversations. EVERYTHING she says is about what he supposedly said and heard by nobody else. I'm not surprised she cried whilst giving evidence, if nothing else, she knew she wasn't entirely innocent, and but for the ineptitude of Jeremy's team her evidence could have been challenged and broken.