But impossible to prove unless they watch his vehicle 24/7

Indeed, and if they dip his tank he's got the stuff that he's paid duty on in there, and as it's mixed with regular diesel they'd have a hell of a job to prove when and/or how much he'd put in

I read somewhere that a small amount of veg oil mixed with the regular diesel (say 25% or so) is actually good for the injector pump, as the veg oil lubricates better than today's low-sulphur diesels, but the mixture is not so viscous to cause problems.

It sounds like a good theory, I might have to chase up some used oil and make a big filter