It is cancer of the lymphatic system, of the white blood cells. Your lymphatic system is a network of tubes and nodes that carry white blood cells. Your nodes collect bacteria and other stuff from your body. This form of cancer is most common in teens and young adults, I have no idea why. You probably already know that there are 4 stages of cancer and stage 4 is the worst. I had stage 3 and I was treated for about 7 months. Lymphoma generally isn't a very deadly cancer. Basically, if you had to choose a cancer to have it would be that one, stupid but true. There are 2 types of lymphoma: Hodgkins and Non-Hodgkins. Hodgkins lymphoma is easier to cure. In my experience the chemo extremely awful, and I had to go to the hospital as an in-patient when I got it since it was so strong. The good thing is that the treatment is shorter than other treatments. She wil probably suffer more than others, but for a much shorter time.
Don't worry about your friend, she'll get through it :)