85-100 rpms is very high, and can hurt knees, too. Without getting all numbery, you want a balance between mashing and mindless spinning. The only time I have ever had knee pain is when I was spinning too fast in a granny gear up the Blue Ridge. By the second day, I figured it out and put it in a slightly harder gear and my knee pain went away.

Likely she'll have to figure it out for herself by trial and error. Let her body tell her what is comfortable.