From my experience with Invasive Ductal Carcinoma, that had no node involvement with a modified radical mastectomy after a failed lumpectomy; my cancer returned just 3 short years later as a chest wall recurrence. Of course everyones cancer is different, but it is a possibility. There are no guarantees that the cancer will not return, but the odds are good and getting better all the time.
All she can do is take it one step at a time and follow through with all of her monthly/yearly tests and hope for the best.
Good luck!
Angi