I had an aunt who had a mastectomy. It's been a long time. But I remember there were exercises she had to do several times a day. It was to help heal or rebuild the muscle. The things I saw weren't hard. She had to stand with her side to the wall and walk her fingers up and down. And, that's all I remember about that.
When I was 14, I had a friend who was diagnosed with cancer and they had to remove her leg at the hip. I'll tell you what she told me. She said that people avoided her. They seemed afraid to come close to her. And, the friends she had that would come over to her avoided talking about the cancer. I guess they were afraid to ask questions. Maybe they were afraid it would make her sad - I don't know. I do know that I wrote letters to her all the time. I told her I was going to ask questions, but if she didn't want to answer them to let me know. She was glad that I was willing to ask and let her talk about her experiences and what was going on.
I know others have said to encourage your friend to talk. But, you have to be willing to ask questions. Ask anything you might wonder about. It will be a great learning experience for you (I'm SO not trying to make this sound like fun - but thru my friends experience, I learned a lot about chemo and ratiation that I didn't before know). But, aside from that, it will help her. It will make her understand that you care.
I wish you and your friend both, so much luck. If you can just be there as a sounding board, from what I understand, that is the best thing.