If he has an original #11 striker it will not work with a 209 primer.
Put a 209 breech plug in my made in 2000 StagHorn in 2002. It has worked well eversince.Sohave several others that i have put 209 breech plugs into.
The "conversion kit" is for pre-2000 StagHorn guns.
http://www.shootersservice.com/cva_staghorn.html