I’ve worked one on one with four clients who have had breast cancer, and they’ve all been very different. Two had already recovered, with very different limitations. Two went through diagnosis and treatment while working with me (taking time off as needed).
I agree with Harris that unless a client is well recovered when you take them on, you need to consult with the potential client’s doctor and/or physical therapist. The amount of reconstruction can affect mobility and the length of recovery. There are also emotional challenges that can make an exercise that would be physically beneficial for the client emotionally difficult for that client.
As with any serious health condition, one size does not fit all.