For the sake of discussion. We are talking about buying a dedicated elk rifle here, not what he already has (which may be "good enough") or for other species. Because of that, I'd bump the power level up just a bit to 300wsm or 7PRC or higher.
Legendary author and outdoorsman Jack O’Connor touted the 270 and slayed big game all over north America, Africa, and elsewhere. One can only imagine what he could have done with the modern ballistics and optics of today in something similar.
The first 2 minutes of this clip with accomplished outdoorsman Randy Newberg pretty much sums up calibers similar to what I mentioned.
Elk Hunting Cartridges and Calibers - Randy Newberg's thoughts
A very accomplished acquaintance of mine in the West has harvested an elk virtually every year for the past 20+ years. Always used a 270 and this year’s was well past 400 yards. Shot placement.
Here, Wyoming elk guide Robb Wiley gives some interesting perspectives in this 5 minute clip on different calibers.
Len Backus and Robb Wiley discuss some of the best elk hunting calibers
And the great thing about a forum like this is the discussion, opinions, and back and forth information to continuously learn (at least for me).