hi...... i generally deal with such patients.....
the first thing you can try with the long AKBK knee braces at night and stretching the hamstrings by placing pillows under the heels.
secondly you can go for the quadriceps strengthening exercises such as the terminal knee extension exercise and the proprioceptive exercises to facilitate knee extension.
you can actually go for surged faradic current with low intensity so as to facilitate extension......

hope these all acan surely gain knee extension in her...... but make sure whether knee flexion contracture has not developed otherwise it would require an arthrolysis surgery....