I will go with the rest on the 25-06 up to the 308. IF your friend handloads, he has a great assortment of good bullets to choose from in all of those calibers which he can reduce recoil as well as have pleanty of punch when it arrives.
On the brake, they are great if you don't mind the additional noise associated with them. My friends 300 RUM is like shooting a 22-250 after he had one installed. It seems the faster you drive them the better they work. He might also incvest in one of the PAst Recoil Shields. I have one for some of the lighter weight rifles I have in the bigger calibers. They sure do help out with felt recoil on the bench.
On the long shots, he will need to do pleanty of practice, to be sure that when the time comes he can honestly be ready to handle it. Pleanty means several times a week for several months out of the year preferably to ranges as far or further than he expects to actually hunt. I would suggest using one trip to work with nothing but a .22 cal rifle to get his trigger control and such down, and the other day to work on his primiary huinting combo.You would be amazed at how much a simple .22 rifle will help you out when you start moving the target back on it. Once you get out to 75 or 100yds with the rimfire your looking at a whole new game. Then when you work with the high power rifle the training helps out dramatically. I have made several shots out past 400yds but only in near perfect conditions, or in a situation where I knew the load and rifle I was shooting inside and out. My 25-06 was one of the most accurate rifles I have ever hunted with. I had no problems hitting yotes between the eyes at over 300yds. My longest shot on deer was just over 400yds down a long narrow pasture. I took close to 5 minutes before I sent the round on it's way. The results was an instant kill as I had waited until I had the exact shot I wanted. THe 115gr Partition enterend the base of the neck, on the front of the left shoulder and exited the back part of the ribs on the right side. It took out the area between the neck and shoulders as well as the heart and the right lung.
My daughter now has my 25 and has used it to take several nice deer along with a few yotes and hogs as well. She would hardly weigh in at 110 on a cold day all bundled up. She would rather hit the woods with that rifle than anything else, knowing that she has the confidence and ability to cover anything we have on our place with it. She also has made one shot lills with it out to over 350yd.
On the other calibers, I have shot hogs out to 500yds using my daughters 6.5x55 loaded with the 140gr A-Max, with the .270 Win using several weights out to over 400yds at hogs, and several variants of 7mm rounds out to 450, then the 30-06, and .308 out to over 400yds on hogs. With the right bullets and putting them where they need to be, they will drop on the spot. Recoil from all of these rounds has never been to bothersome in the field, but a couple of them really do rock me on the bench. My little Ruger Compact in .308 only weighs 6.5# and has a 16" barrel on it. IT knocks the snot loose from me when I shoot it from the bench, but it has accounted for three of my biggest hogs, as well as taken two of them at ranges over 300yds. with one shot using factory Remington Core locks. If all of the above will drop a feral hog topping upwards of 300# they will definately put a deer down with the right placement.
Good luck and remember no matter which caliber he chooses, it only matters if he can reliably hit what he is aiming at. Other than that he might as well just take the camera. I did not get up one morning and go out and start shooting stuff at longer ranges than some, I grew up with it as there was no way to get closer where we hunted. It was flat hay field in heavily hunted farmland, and the deer didn't walk out and look the other way when you tried to get closer. I have hunted since I was 6 and now at 43, I am still learning things about it. It has always been my goal to get as close as possible, but I also wanted to learn to get the most out of my equipment for those times when all else failed, or there was no other option.