That sounds traceable...
She should have asked him with a gun pressed against his temple.
By you.
There's a strange thread of decisions by this guy... selling up and emigrating to Thailand without a Thai spouse to drag him there, and evidently selling up a 6-figure sum of property to go there ...hiring untested locals to do a job he's apparently experienced enough to do himself, and probably better (or to western standards) ...not supervising the job, day by day ...then complicating the payment, perhaps not pre-agreeing anything in detail, belies a lack of knowledge of the risks and the way things happen in the country... I make a crap Columbo, but it seems like he didn't research his move to Thailand, and moved a bit hastily, and expected things to work like in a developed country.
Perhaps the lesson is, if you hire Thai contractors and they do a shit job, shoot them, and dispose of the bodies discretely over the border or in the sea.