Cosmoline....
Eghad has a good suggestion - ask him what he's learned and what he would do differently. The answer is important. If your buddy is really ashamed of it and indicates that he's learned a humility lesson, I think you can take comfort in that and he'll be much more careful.
On the other hand, if he laughs it off or makes light of it, you may have a problem. In this case, explain just what you did here... how can he be trusted not to do it again? How can you relax in a place where someone who does that (ND's) laughs it off like dropping his beer on the carpet.
In either case, occasionally (once a month) ask him if he's killed any more floorboards. Peer pressure over doing something stupid is one thing that will force most people to ensure it doesn't happen again.
I had an ND once with a .45 at my Dad's house when I was in my 20's. Put a hole in the floor right next to the bed.
His first concern what that I wasn't hurt. Then he told me - "I'm
so disappointed in you." which really hurt.
I became much safer in handling thereafter, especially since when we'd go golfing together every month he's ask if I'd shot any more floors. He kept it up for about 6 months until he was sure I'd learned my lesson.