When I went to my GP with a breast lump he said to me 'Don't panic until someone tells you to'....and then they did. So the first thing I will say is that it may not be breast cancer.
If this is the diagnosis, she will probably feel shocked and scared and a hand to hold will be very reassuring. She is likely to be told what the suggested treatment could be. Depends on what size and where, but the options for surgery are a lumpectomy (part of the breast containing the cancer is taken away) or a mastectomy where the whole breast is removed. There will probably at the same time be a sentinel node biopsy, where a few of the lymph glands under the arm are removed at the same time as the surgery. If there is a sign of spread, they will mostly or all be removed.
After surgery she will get physiotherapy and exercises to do to get the arm moving again. Then there is about a 10 day wait to get the results as to the type and extent of spread, if any. This all determines the next stage of treatments: chemotherapy and/or radiotherapy and possibly hormone treatment for about 5 years.
The key person she will have to help her, and that you can relate to, is the breast care nurse who will be there to support and guide her through the process. You may meet her tomorrow. Mine was brilliant, holding my hand physically and emotionally through the whole process.
As for your role, having someone with her both at the crucial times of test results and treatments is so important. She may feel rubbish at times, and will likely be scared of what is in front of her. Just listen and show that you care. Encourage her to be open about her fears, but also help her to work with treatments and think as positively as possible. Be guided by the BC nurse. Also, as other people have said Macmillan have lots of information and support systems to help as well. Do encourage her to find out about these.
In my own experience, I was too busy getting through the rigours of the treatment to initially feel wobbly emotionally, but about 6 months down the line I really needed someone to be there with emotional support as I felt washed out.
Your friend is so lucky to have you. It is good of you to be taking the trouble to find out as much as you can. I'm sure other GNers can give you more of their experiences. I know there are several who have been through BC treatment.
Good luck for tomorrow.