60 yo. I have a knee that reminds me when I am trying to push too big a gear...the smaller gear (smaller chain ring, larger cassette cog) slows me down a little, but worth it to get the distance. Seat position can aggravate my knees as well...specifically too low or too far forward.

Your friend may find an easy warm up helps too...if I try to go too hard in the first 10 miles, it substantially shortens my distance for the day.