I've been working for gun stores for about six years now, and I've heard every reason in the book for not using credit cards for a gun purchase. Your friend's story is one of the many. A lot of people also think the government will never find out they own guns if they pay in cash. Which I never really understood, but I guess some people don't get how the whole process works unless someone walks them through it. Where do you think the 4473 eventually goes?
I try to pay very little of anything on credit. Usually just gas if I can get away with it, and a few things that require a credit card to be taken out every month (Aflac, my gym membership, etc.). But I only do that because it keeps me out of financial trouble. You lose a lot of temptations when you have to handle the cash right there and not push it off to worry over later. And I don't even own a debit card.
But ya know, my tin foil hat did blow off some time ago...