Originally Posted by bamazav
This is one of those "depends" questions. Depends on if she is new to lifting, lifting a while, preexisting conditions, etc...
If she is new to lifting, I would start with lunges or Split squats. Leg extensions are limited in their effectiveness, IMHO. Machine squats can be hard on the back, and my experience is that if they won't do BB squats they probably won't do Smith's either. Leg press can be helpful, if done correctly. My newbs usually start with Goblet Squats, step ups, split squats and Lunge varieties.
Goblet squats, as suggested by Bam, would actually be an excellent replacement for back squats.