He wouldn't be able to enter UTMB right away because he doesn't have a "UTMB index score". Same thing with Western States....you have to have a Golden Ticket entry by earning one first (as an elite). Otherwise no entry.
Comrades..sure....less barriers to entry. Although traditionally South Africa doesn't always invite all the Kenyans that want to run (I've heard) and generally you have to register with a local club team...which can get political....
What I'm saying is that there are actually a lot of barriers to entry just to make it to the starting line of a lot of the top ultras. It generally makes it so the competition is less deep (compared to really any big road marathon).
I do think he (or basically any sub 2:10 marathoner) would for sure do really well and destroy a course like Western States 100 though. Sure there could be a learning curve and it's not a 100% guarantee ...but more often than not ultra success correlates directly with your marathon fitness/times. UTMB is quite different than WS100.... it probably has more in common with Zegama honestly although it really depends on the weather on the day (mud etc).