Of course the safe answer is "it depends" but I can share my experiences. I think you mean stage 3 instead of grade three, that means that the lymph nodes were involved, as they were. That means that it may metastacize because it spread to the lymph system. From what I understand, there's a bc pathway when it spreads and watching for symptoms in the back and the head are pretty important since the brain and bones are in the pathway. She will have to be checked by her oncologist regularly for tumor markers. not sure what exactly those are or what the numbers we want are, someone else will have to help there.
As for life after chemo and radiation, I'm pretty much back to normal, except I never know if I have it again. It's a free-floating fear that rears it's ugly head occasionally. Had a bite on my neck or a hive or something and first thing I felt was the "lump" Before I could process that it itched and was of the allergy nature, I thought cancer's back. I can tire easily at times and have to get bone density tests annually. I was 38 when I contracted stage 2 cancer and now am 42. Still cancer free we think. No node involvement.
Psychologically, I have a new lease on life. Fortunately for me but unfortunately for my wallet, I survived. My first bit of advice when diagnosed is watch the spending, you're not dead yet and you may be chronic not terminal. I agree it's not the death sentence it was, but that doesn't really stop the fear and knee-jerk reactions, it just helps combat them. Lots of self exams and getting a breast exam whenever I can is something else I do. The hair grew back. Don't know if I can ever give blood again, though. What else, anyone?
"Our greatest glory is not in never falling, but in rising every time we fall." -Confucius
Don't knock on Death's Door. Ring the bell and run. He hates that.