I was first on the list and sent home that afternoon. She will need regular pain relief. On the way home she will need a cushion and padding for the seat belt.
I had a single mastectomy with axillary clearance, so I have a scar from my sternum to my armpit, and my drain was at the side. It was sore but not painful (for me). I didn't have that much drainage and my drain was removed by the community nurse after about ten days. I had visits every couple of days to check my drain, but if I had needed more frequent visits I could have had them.
I had a LOT of nerve pain in my arm after my surgery, every little touch was agony, even fabric touching it was agony, and I needed gabapentin for the pain, but it did resolve after a couple of months. It is more likely for somebody to have that kind of pain after axillary clearance than just a simple mastectomy. It is 2 1/2 years later and I have no more pain but I still don't have full feeling in my chest, my side or my upper arm.
After surgery it is really REALLY important that she does her exercises. They hurt, and she will need some support to encourage her to do them.
Listen to her, don't assume you know what she will want or need. It is incredibly frustrating when you, the patient, are told what what you want and need by somebody else. You can encourage her to eat nourishing meals and help to provide her with them, but if all she wants are pringles and Coca-Cola it's better than nothing (that was me). If she says it hurts, then it hurts. If she says she's fine then use your own judgement, but for goodness sake don't tell her she's not telling the truth!
She will need some help with the physical stuff until she heals. I'm not sure how much help you are going to be able to provide yourself, try to encourage her to take whatever help is offered. She won't be able to drive if she's on strong painkillers, so if that is the case she will need help with transport to and from appointments.
And it's not just physical pain she may be suffering from. Losing a part of you that is such a major part of your femininity can have huge psychological implications. Encourage her to accept counselling. I was given 6 sessions of counselling by a local cancer charity and I needed it. They also gave me 6 massage sessions, provided by a therapist who was a specialist in cancer patients. I found being able to visit a specific cancer centre (not a hospital centre, a supportive and helpful charity) so very helpful, but everyone is different. It may not be for her. but
And surgery, plus maybe chemo and/or radiotherapy is not the end of it. There may be on going treatment that continues for years that will have its own side-effects and problems.
This is what happened to me and what helped me, but there is no 'one size fits all' approach that will be the same for everyone. Allow her to be herself. Listen to her.