A couple questions as it could easily be ammo related or gun related. If you took the cartridge in the problem chamber and put it in another chamber, would the gun function properly? If so, its a gun issue. If the problem then followed the cartridge, its an ammo problem. Likewise, if you completely unloaded the 686 and reloaded it with completely fresh ammo (ammo not having been loaded in the cylinder) do you have the same problem? Did you mark the chamber to ensure the problem is always on the same chamber?
Ok, a few more questions on ammo. Were you shooting factory or reloads? Lead or jacketed/plated bullets? Were you shooting .38 Special or .357 Mag, or did you shoot .38 Special then .357 Mag? If you shot .38 then switched to .357, the .38 can leave a fouling ring in the front part of the chamber that can prevent the .357 round from fully seating in the chamber thus leading to the issue you described. Likewise, if you were shooting a lot of soft lead and the b/c gap is too tight, it can tie a revolver up as well, but it would probably be on more than one chamber.
Sorry, too many variable in my mind to know if its a gun or ammo related issue. Also, did your friend fully clean the 686 before taking it to the range? New firearms should always be fully cleaned and lubed to get the non-lube preservative off and to help flush out any small pieces of metal and manufacturing crap that can still be on the firearm. A little metal shaving in the wrong spot can easily tie up a revolver.