I have owned about three over the years. Value depends largely on the condition. These were offered only in the 1905 sporter configuration, not in the 1907 (AKA Deer Stalker) which were .280 only, or M10s which were .280 or the 1910 which were .303. The 1905 sporters came in two grades, R, which was a basic finish rifle (but still pretty darn nice) and an E. The three .35s I have owned were all "E", which were finished to a higher standard. They had nicer wood than the "R", with chequering and usually a set of express leaf sights on the barrel instead of the usual Winchester type tangent buckhorn. Production figures are not known as the Ross records were pretty much all lost, but .35 are definitely less common than the .303. I have seen nice .35 go anywhere from $900 to $1500 in the last three years, but again, outside condition, bore condition etc rule the roost. So, even though a .35 is rare, if the wood has been buggered with, and the action reblued with the bore pooched it is a $200 gun max.
Ross collectors are a small niche group compared to most other groups, and sporters have never commanded as much interest as the military rifles. Regardless, I consider Ross factory sporting rifles to be elegant pieces of art, even the basic ones, and a well cared for one can shoot wonderfully and be smooth as butter to operate. Photos of your friend's .35 would be most welcome.
Ed