30-06 would be my vote. I have a 7-08 and a .270win but if I could wave a magic wand and make them .308 and 30-06 I probably would, but what I've got is certainly more than adequate, but I think .308 and 30-06 factory ammo is easier to find as are reloading components, surplus military ammo is also available and they are just about the most common CF hunting cartridges in existence.
I love my .270, it's laser accurate and might even be ideal for me since all I'm likely to shoot with it is whitetail deer but if your buddy is after Elk, he might be slightly better served with the heavier pills the '06 can accommodate.
Although if he found a good .270win, it's about just as common and available in stores as 30-06 and from what I understand, .270 is considered by many to be a handy lil Elk cartridge..... alot of non magnum long action cartridges seem to be dropping in popularity with each passing year, but I imagine the .270 and 30-06 will be with us for a long time still....