30-06 or greater........reliability is most important
Yep. I'd not buy a new rifle just for the hunt (if he owns a 30.06 or 300win mag etc). Unless of course, he wants to do so - perfectly understandable.
I know I'm preaching to the choir, but I'd encourage him to take something he's amassed some time/familiarity with (if possible) over something new that likely won't get shot a ton ahead of time.
When I go back, I'll likely take a 30.06 or the 35 Whelen (again).