My friend has been having problems with her ligaments in her left knee. Is there any acupressure point I can learn to help her? She may have to get surgery for the second time. Keep in mind I am only 14, and she is about 2 years older than me.
At that age with such complications she should see a fully trained and licensed acupuncturist to help her. Acupressure alone will not be enough. That said, some of the main points used would be on our acupuncture for arthritis page (near the bottom there are sections for individual joints). Even though the page is for arthritis it is applicable generally to all parts of the body.
There are many things that can be done by a professional to help her, possibly enough to avoid another surgery (without knowing her whole case it's hard to say). You would want to see an acupuncturist who also practices tuina (Chinese Medical Massage).
Bless you heart for trying to help your friend. A professional practitioner is the best person who can evaluate what is the cause of your friends pain. I would not suggest doing a trial and error approach as you may end up doing more damage than assisting.
Given your experience and good intentions to help, explore energy healing. As you have mentioned, she has a medical practitioner treating her already. Adjunctively, it will not hurt if you give her some energy healing.
Your best bet is to go on YELP and search for a good acupuncturist with reviews for pain.