I think that if you are shifting the work load, using different positions and higher resistance, you are definitely working your glutes, biceps femoris, etc. (That's the big leg muscle in the back - I'm married to a painter who teaches anatomy drawing - "Hey, honey, what's this part that's sore called?" That's your biceps femoris, it's looking great from all the cycling, by the way")
The day after a great strength-building spin class, I have a kind of good "burn" in all those butt/back of thigh muscles. It's a good question for your instructor, too. Maybe your friend needs to alter something in her workout. I see a number of people who attend class after class and never seem to touch the resistance knob (or break a sweat...)