The various other causes of knee
locking are torn
medial meniscus, mis-aligned knee
cap, osteoarthritis, knee
lateral meniscus, osteochondritis dissecans, recurrent patella dislocation, chondromalacia patella, knee alignment problem, knee fragment or knee cartilage disorder.
Since pain intensified after a fall, maybe she has injured a ligament. Please discuss with her doctor. It may be a good idea to do a MRI, if that is possible. Take care!