You seem to have drawn several comments, but very little useful information!

I am only guessing that if there is a lug and proper flash hider, a standard M-16 bayonet should fit. Bayonets are a good CQB weapon if you run out of or low on ammo or time to reload. May be useful in a zombie wet dream, but of little use in civilian life! I recommend he spend the $40-60 on ammo or batteries for his doodads!

Ivan

P.S. You didn't post where he is located, do they have an octogenarian infestation there also?