Could it be more productive to work independently after PhD?
In academia, he might have


more exposure to interesting ideas (especially cross-field pollination),
better access to students and colleagues to exchange and refine ideas with, 
as well as external pressure to be productive (which might be a good thing.) 


While administrative work has never inspired me to have an interesting idea, teaching and mentoring students certainly has.

It all depends on the context he's able to set up for himself as an independent researcher. If he works better in isolation, maybe he'd be better off. If he can afford to hire other researchers and create an interesting intellectual environment, that might help.