That's a killer deal! Mags are dirt cheap but if the gun is NIB with both stocks it would bring $2200-$2400 easy I think.
Look over some of the auctions and check out the pics. Most of them will show pics of the markings on the magwell with the date codes. If the markings on your friends rifle are like those it's likely genuine HK.
If that scratch is any indicator to the condition of the rest of the gun I wouldn't call it NIB. Still a great deal though, if the rest of the gun is good and the bore is excellent.
Looks like a real mccoy HK91 to me.
A forum dedicated to and laser-focused on Heckler & Koch firearms, Heckler & Koch accessories and the owners and enthusiasts that love them! Come join the discussion regarding HK pistols, long arms, NFA arms, HK accessories, HK history and trivia and more.