Originally Posted by devilshaircut

Cog is cheaper, yeah, but if all he has is a 42t chainring, I'd suggest getting a higher tooth count one just to have. Personally I have 3 spare chainrings and like 5-6 spare fixed/free cogs.

And as stated earlier, he wouldn't need a wrench to change his chainring. I assume if he only has the one chainring, he probably doesn't have many tools either.