There's an interview with the PI, David Marshburn, here
Stepmother of Lucas Hernandez jailed after boy's body found
I've only listened to the first half but he kinda gives an explanation. What he implies is that he has talked to EG about various locations and watched her body language and listened to her speech patterns. So maybe he talks about the sea and she's relaxed, he talks about a bridge on a country road and her attitude changes to the exact opposite.
Somehow he seems to have turned that into her going out there with him and finding the location, which she only knew by rough memory. He hasn't talked in that interview about methods he might use to achieve that end.
The tactics he's used are tactics that are available to LE/FBI, but he managed to insert himself as an unbiased shoulder to lean on, something that LE can't do very easily. LE are always there with handcuffs waiting, but if you look at his picture on that interview, he's just like a bloke down the bar who you can sit and have a beer with and open up to and he's not judging, he's just talking.
JH is getting a lot of bashing, I don't deny that some of it's deserved. But, he wanted a PI. It's possible that him trying to keep on good terms with EG has helped that as Marshburn tried to access her emotions that having JH around could act as a catalyst to the reaction that Marshburn is trying to set off (using chemical science as an metaphor). If JH hadn't been there it's possible EG would have found it easier to resist Marshburn's techniques.
If it goes to trial, then hopefully we'll find out a little more, but I hope Marshburn doesn't talk too much about his 'magic'.