The ACL injury, or a rupture of the anterior cruciate ligament, is a common knee injury. It can happen while you are playing a contact sport like football or rugby, or as your friend found out, just by running. It can be caused by changing directions quickly or coming to a quick stop, for example. It also seems that more women can develop a ruptured ACL than men.
If a knee injury occurs, early symptoms of an ACL rupture can be a popping noise coming from the knee, severe pain, and swelling. Of course, care should be taken not to make the injury worse. You can use ice and elevate the leg to reduce swelling of the knee. If permitted an NSAID (non-steroidal anti-inflammatory drug) like ibuprofen or naproxen may help too. You should get the knee checked by a doctor to evaluate the seriousness of the injury. You may need to wear a brace or splint to keep knee movement to a minimum, and you may need physiotherapy to help your knee regain full range of motion.