ra4000,
Kind of a tough call. While he'd have his choice of regionals if he wanted to go that route, it's hard to say if having time in a regional airplane (and Part 121 experience) vs a G4 would improve his chance at a major. I'm not qualified to have an opinion on that. I know the majors hire corporate pilots but I don't know how many.
I do imagine the paycut at a regional would be painful for a while ! Has he considered places like Frontier and Spirit ?