Sorry to read that about your friend.
The physical and mental impacts, she will have to face, will depend mostly on the stage of the disease, as well as the kind of treatment. And of course her current fitness level. So this is quite individual. I highly recommend talking to her physician about the training.
Nevertheless, there is good evidence that physical training is very beneficial for breast cancer patients. This was also shown in a big Swedish study:
I’m currently not working with cancer patients. But back in medical school a prof., some other students, and I were “training” with cancer patients. This was only fast walking, but the patients really enjoyed and benefited from it. Also, the conversations were very helpful for them. And despite the serious diagnosis, we had a lot of fun.
All the best to you and your friend!