Bryan:
Similar experience. Had OHS to replace aortic valve and ascending aorta at age 51, with a history of back issues.
My back was very sore for about 8 weeks, and it took almost 6 months for all of the soreness to go away.
Got the same response from surgeon, that position on table during surgery and the mechanics of spreading the sternum led to stress on the back.
Found that as I went through the cardiac rehab exercises, improvement came quickly. Did have a month interruption in rehab as they figured out how to work through the start of COVID restrictions.
Hope she feels better quickly.
Keith