Usually, yes.
The fact he has a job is great. Making more money = less job security. The more you make, the bigger the risk of getting axed. Also, the more responsibility, the better the chance someone's small fuck up leaves his kids without a pot to piss in.
I'm sure Will would do great, but there's more than just his job that he'd be changing.
I agree with all this. But, there's ways to do that, without adding a risk factor. Except for the 8 months I was away from my current job, I've been working here a total of 4 years. I have no title, but I make almost as much as the supervisor's and have been asked many times to fill in for other departments, train new employees in other departments, and do some of the supervisor's duties when they're not here. I have phenomenal job security, and wouldn't change a thing.
Your situation and his are apples and oranges, similar but different, he's staying with the company that already likes him and being promoted from the inside so he already has a rep going forward. Plus his new job will have a higher pay than his current one true, but it will be in line with whats expected of the position and commensurate with his experience and track record within the company so the extra money isn't less job security. They have a job that pays X dollars, and some one will -no pun intended- have to take it, might as well be him the guy they know that does great work to take it rather than some no name.
If you he has the talent and the skills -and I believe he does- he'll get the new customers and employees to like him like the old one do. For me, personally, I always take new opportunities when they come my way and try to do my best. He doesn't have to follow my advice. I wish him the best of luck though.