Sorry for the misunderstanding simon I thought you borrowed his gun / rifle and used it without his supervision my mistake simon sorry.
I absolutely would not borrow anyone's rifle (even if the calibre matched those [in this case just the one] on my FAC) without them being right there with me, on site, for the instant of the shot... after which they would have to take their gun back. For one thing, if you were borrowing someone's gun without them being present, and you committed a crime with that gun, it'd be traced back to them.
I wouldn't let anyone borrow my rifle without my presence, would want to borrow anyone else's likewise!