Your expert -- who is it - Plait? -- does not understand McCanney's model.
The x-rays are not produced because of high heat. I've explained this in simple language.
The x-rays on the sunward side of the cometary nucleus are produced because energy is conserved. When the electric current from the sun reaches the comet it must slow down because the comet is moving very fast -- not sure how fast but probably 20,000 miles an hour at least.
As the electric current slows down it must give up energy - which it does in the form of x-rays.
Incidentally, Dr Dwyer also discovered that lightning also produces gamma rays -- and for the same reason.
I would not be surprised if astronomers find that comets also produce gamma rays -- and by the same mechanism. Maybe they should look.