I use the 7x65r and love it, and have used the 7x64r as well. It's a very versatile calibre with factory loads from fast flat shooting 123gn bullets - ideal for chamois, and long range small deer to 177gn bullets at c2650/2700 fps thumpers for big animals. It will happily take anything in Europe up to and including elk. The 8x57 has the thump for big game but does nt have the longer range flat shooting capability of the 7mm. My main comment though is that you friend already has the 6.5x55 and that prob does 90% of what a 7x64 can do. They both have long bullets at good velocities that penetrate well, yes the 7x64 is a bit more powerful but its not a huge jump up.
At the end of the day though, and as Muir commented, much of the choice will be down to the actual rifle.
from the Norma.cc website similar also RWS website
8x57
8x57 JS - Norma
The 8mm Mauser was adopted by the German army in 1888 and in the following years by a number of countries like Czechoslovakia. Poland and China. The designation “Mauser” is actually a misnomer as the cartridge was developed by a German military commission at Spandau Arsenal for a forerunner of the famous Mauser rifle which was adopted in 1898.
The “J” in the name stands for “Infanterie”. The “J” designation is due to a mistake deriving from the previous use of Gothic letters in Germany and has no significance regarding bullet size. But in 1905 the German army switched from round a nosed 226 grain bullets to a 154 grain pointed boattail bullet, and at the same time the diameter of the bullet was altered from .318” to the present standard of .323”. Accordingly the “S” - short for “Spitzer” - means that the barrel is made for .323” bullets.
Actually it is quite rare nowadays to find a bolt action rifle in the old “J”-caliber (.318”), but old combination guns are still around and a good many of them were made for the 8mm JR. To further complicate the problem many combination guns have only the bullet diameter (8,2 mm) stamped on the barrels regardless of the fact that a large number of 8mm cartridges have been made with very different sizes and shapes of cases. If the least in doubt do ask a qualified gunsmith before trying to fire any cartridge in an old weapon - even from the package that came with it.
The 8x57 JS and the rimmed version 8x57 JRS are both very flexible cartridge that duplicate the performance of the .30-06 and similar cartridges. Today not many bolt action rifles are made in 8x57 JS, but a good many double rifles and combination guns are still being chambered in 8x57JRS. However as the latter type of guns has a weaker action the loads should be reduced in order not to exceed the pressure limits.
and the 7x64
7x64 - Norma
Wilhelm Brenneke was a brilliant German cartridge designer and still famous in Europe for his development of slugs for use in shotguns. He also constructed a very modern line of rifle cartridges with the most popular bullet diameters from before the turn of the century, but with a larger powder space than the then “standard” length of 57 mm.
The first was 8x64 from 1912 which was designed in a an attempt to get a government contract for a more powerful military round than the 8x57. The attempt was in vain, but as the 7x57 was already popular Brenneke also constructed the 7x64 in 1917 - 40 years before the almost identical .280 Remington was introduced. Designed solely as a hunting cartridge the 7x64 is very widespread in Central Europe as several countries here have banned the use of military calibers for hunting.
To the handloader the 7x64 is a very versatile cartridge with bullet weights available from 80 to 180 grains. You could call it a European .270. As the powder space of the two cartridges is almost identical, the differences in performance are purely academic when it comes to bullets of equal weight. However due to its slightly larger bullet diameter - .284” compared to the .270’s .277” - the former has a slight edge when heavy bullets are needed for large species. The 7x64 is adequate for all species on the Northern hemisphere - including the big bears, provided a heavy bullet of good construction is used.
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8 x 57, a very good and relyable round! Go for it! 
