Without mentioning the unsafe gun-handling, tell the dad the neighbor kid is invited over for BB gun practice the next time your nephew is over. For example,
"Say, Jim. I saw your Danny has the same BB gun I bought my nephew. Why don't you and Danny come over the next time Nephew is here and we'll have a little practice shoot. Let the kids get some practice under adult supervision, that sort of thing."
Only if dad declines, mention the unsafe handling: "OK. One reason Iwanted to ask is that I noticed heforgot his safe handling methods last week whenhebrought his BB gun out to show us."