Rarely, it is due to blocked or pinched popliteal artery that runs along the back of knee.
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, 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.