What muzzleloader is he shooting?
No, a centerfire scope is fine. My 300WM kicks a lot more than my T/C Omega, no reason a scope good for the former wouldn't hold up to the latter. Loaded to 110gr I would say it kicks a bit harder than a .308.
I've shot $200 scopes on my MLs for years, they hold up pretty well. Finally killed a cheap $100 Chinese scope after 4 years of rough use.
Most of the newer MLs have minimal blowback. For instance the Omega and Encore have essentially zero that escapes the action, I never see any on my scope.
I cut my teeth on iron sights and recently bought another ML so I can go back to them for closer-range hunting. I think they're a great way to master the fundamentals of marksmanship. While you obviously don't want to lose deer, I'd encourage him to work on his skills in that area. A scope isn't necessarily going to be an easy fix.