I also agree with Tulip. 85-100 is high. It's what us "cyclists" aspire to, but for a relative beginner riding with the kids around town it's going to hurt. (I spin at 75-90 rpm and I'm actively trying to increase that, but that's as fast as I as a cyclist can spin right now without hurting myself).

I wouldn't give her numbers. Instead tell her to focus on keeping a steady, circular pedal stroke that feels comfortable to her. She'll very likely find something in the 60-80 rpm range, and that is fine. Yes, shift accordingly, which means, yes, she'll be shifting a lot, especially at first.

Also a X 1000 on going and getting at least a basic fit to get saddle height and fore/aft positioning down. If she's using clipless they should check her cleat positioning too as that can exacerbate knee issues. When she goes in for a fitting she should let them know just what her knee issues are and whether or not they are currently flaring up on the bike. Try to find someone that knows how to work with such things (a good fitter will). Depending on your area that may or may not be a reasonable task at a reasonable price range (and it would be a bit ridiculous for her to pay for a $350 dynamic fit at this point, obviously), but worth a try.