It should (should I say) just sit in the bottom of the pan and cause no trouble. There is a screen over the oil pump pickup so it should be safe. It's pretty unlikely the screw would jump up out of the pan and into the crankshaft or anything.
Next time he changes oil, tell him to do it when the engine is hot, just back from a drive. This is a sucky job when everything is hot, but the actual recommended way to change oil anyway. Reason being is that oil flows more quickly when it's hot so it's more likely hot oil will carry debris and suspended particles out with it as it drains. After he's drained it out, pour the oil through a screen and odds are he'll recover his setscrew.
11. If thou be not absolutely sure of thy facts, thou shalt Google before posting thine answer.