Elm responds so well to toasting, you'll probably crank that elm bow from 45 to 65#, and maybe that's out of your dad's comfort zone (it would certainly be out of mine for accurate shooting). So unless you're really concerned about speed, I would just leave it as it is.
Actually, ion the last elm bow I heat treated with a heat gun, the back also started to show transversal tension cracks. It's still shooting, but there's dead mass slowing down the bow now. But I don't feel like chasing the growth ring below, or worse, backing an elm bow ?!
Joachim