In my opinion, I think you're friend is the one who's bike maybe set up wrong. If s/he is only feeling it in the quads, then his/her saddle position maybe too high and s/he tip-toeing on the clips, therefore not being low enough to employ the hamstrings on the pull back of the down stroke. Some cyclists ride with the saddle way too high and they're hips are actually rocking from side to side on the bike. Sure, you can ride like that but it's not using all the muscle groups efficiently. Cycling uses all muscles in your legs, from the calves, glutes, inner and outer quads, and the hamstrings, not to mention the upperbody as well. Some muscle groups are used for the forward (push) part of the pedal stroke and the others are used for the back (pull) part. When I ride a short distance or longer rides, all the muscles in my legs and calves feel it. Maybe it's because of the type of bike I ride, but I don't think so. Seated, standing, hill climbing or spinning on flats (I think) works it all.