RE: Favorite Russian Boar Caliber
35 rem is a good caliber, but it isn' t a real flat shooter. Where we hunt boar in CA. we often have to shoot out to 2-300yds. Definitely not a job for a .35. I use my .270 or 7mm mag. I have the very gun you mentioned in .35 rem that I hunt deer in NY with. Our shots there rarely exceed 100yds.
Maybe the way to approach the hunt for his gun is from the perspective of what else you and he may hunt in the future. If you have elk in plans someday, then a good old trusty 30-06 is just the ticket. About the same recoil as the .35 but much better ballistics.
Just my .02 worth