The various other causes of knee locking are torn medial meniscus, mis-aligned knee cap, osteoarthritis, knee arthritis, torn lateral meniscus, osteochondritis dissecans, recurrent patella
dislocation, chondromalacia patella
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!