He will get diamond status since RCI gives the highest level that either spouse has to both of them. However I am not sure when he'd get it since he has never cruised with RCI and therefore has no C&A membership account. She will likely need to call the C&A desk. I'd think the worst case scenario would be that on their first cruise they would go to the loyalty ambassador and they would activate an account and give him a new cruise card. But maybe for married couples they will open an account before the first cruise.
But the important thing is he won't have to work his way up to it. I just made Diamond Plus and my wife's account was automatically upgraded even though she was not on the cruise and didn't get the points.