As roklok said, if your friends gun is a Guide gun it likely is a Marlin with an 18.5 inch barrel. I don't know the price of Hornady verses Garret in your neck of the woods but if you/he don't reload then I would say the Hornady Leverevolution ammo would certainly get the job done. It looks like it only comes in 325 gr. but with 2158 ft lbs of energy at 100 yards and 1516 ft lbs at 200 yards, it will certainly do the job. I load Hornady 350 gr RN and Remington 405 gr FN in my 45-70 and both work just fine. If you were hunting dangerous game then you certainly would want to use the Garret ammo and it would certainly work very well on Elk. Garret ammo will definately penetrate further but I don't know if it is needed for Elk. roklok is correct about the Garret ammo in africa etc. it has taken them all.
I would use the Hornady Leverevolution ammo and not think twice about it. JMHO