It could be several things
1. IT (iliotibial) band syndrome: Touch tissue that is from your hip to the outer part of your knee. When you over-exercise, it can become inflamed. Causes pain on outer side of knee.
2. Meniscal Tear: When cartilage rips. Pain and swelling are common symptoms.
3. ACL- Anterior Cruciate Ligament. Mostly seen in athletes.
I'm pretty sure her problem is number 3