I’d like one rifle to be all he needs even if elk at 400 yards is a likely shot.
Well, you've just described a 270 AND a 308, 7/08, 280 and 30-06 (and anything that fits in between those). I think starting a new hunter, irrespective of size or age, with a magnum is just setting them up for failure/problems. Choosing to shoot a lighter recoiling rifle does not make one less of a man/woman. Arguably, it displays one may even be smart. Good luck in your quest and kudos to you for helping another prospective hunter.
I think a lot of us agree totally with your post. I, personally, would never start a new hunter/shooter off with a 300WM. i dont care how old, big, strong, small or "manly" said shooter is. It's just asking for trouble, as the new shooter is likely to develop a bad flinch and bad habits and probably not even enjoy shooting. Like Brad asked the op, how many elk has he killed with a 270? The 270 is an excellent all around cartridge choice, assuming you use the proper bullets for the job.