The most likely scenario is that you'll lose the bet. So the "buy the bottle" advice is where you should start, so things end well and on a good note.
However you should do your best to try to win this. And also make sure that you and your buddy have the most fun you can in the process, since odds are he's going to win the bottle - might as well have a laugh with him and share the rum.
Just to let you know what you're dealing with - I have a purpose built target rifle that shoots about 1/2 MOA at that distance. If I was to sight in my rifle at that distance until I was happy, then hand it over to you to shoot prone, with a bipod, and a rear sandbag as a rest, you'd have a good enough chance at hitting a pop can that it would be worth trying, however it would be far from being a "gimme". As an experienced shooter, I'd put my odds at hitting a pop can at 500 yards given three shots and a just-sighted-in target rifle as perhaps 70%, but probably not higher than that. If you asked me to shoot it offhand, I'd put my odds at about 2%-5% (seriously).
If you have good handloaded ammo for your scout rifle, and if you shoot it prone off of good F-Class equipment (rear bag, front bipod or a front benchrest style rest), you're going to have a rifle that shoots anywhere from 0.5 MOA to 1.5 MOA. To get one hit out of three, in this configuration, will be challenging. Stand up on your hind legs and shoot and it'd be more a matter of luck than any reliable repeatable shooting feat.
Regardless of how you approach the firing of your three shots, you should start by getting your rifle as well zeroed at 500 yards as you possibly can. Use good prone F-Class or benchrest equipment and techniques, and shoot at 500 yards until you are happy that you are within one scope click (1/4 MOA, which is 1.25" at 500y) of being perfectly adjusted. As you can see, a single scope click is more than half the width of the can.
Simply standing up and firing three shots at a single can is so unlikely to hit as to perhaps not be worth trying - or at least this shouldn't be your only approach to the problem.
I would suggest that your principal approach to the problem be to cheat. Don't hide the fact from your buddy that you are cheating, choose the method of cheating that will be the most fun (e.g. shooting at a case of pop stood on end sounds promising to me - that's a pretty worthy offhand shot at 500y, I'd say I'd have 30%-50% odds of hitting it given three attempts). After you've cheated and (hopefully) won the bet, you should probably try doing it in the manner in which he understood you would be doing the shooting - stand up and shoot three shots at a single pop can. You'll almost certainly miss all three, but it's worth doing. You should also pass the rifle to him and have him fire three shots standing at 500 yards too, so he knows just how hard a shot this is. Then regardless of whether your cheating has helped you win, you should give him the bottle of rum with a smile and a handshake (unless you've somehow managed to win without cheating!).
