I would say that I had very early breast cancer (or pre, DCIS) with some tissue removal about 4 years ago on my L breast. The area in question was the most lateral area, deep in the breast. My scar is small but runs on the lateral aspect if the breast nearly right under the armpit. I did not have any reconstruction nor implants etc (nor any further treatments). Thankfully all scans since that point have been negative for recurrence (2x yearly)
Now that I gave that info, I would add that I have had recurrent shoulder issues starting several months after the surgery on the R side and basically flipflopping from side to side since that time. I feel my issues them from initial overcompensation and scar tissue. I had breast massage (which is quite helpful), had/went several times for PT, etc. I currently Olift but did/do have to take off time periodically. I have also learned (thanks to my coaches) that pushups, pullups and bench presses are exercises my body does best without. It has taken me a long time and my issue wasnt as severe but I am learning to listen to my body, FINALLY!
Best of luck with her recovery. She is very fortunate she has you as her support person.