My Daughter In Law Has Been Diagnosed With Triple Negative

My daughter in law has been diagnosed with triple negative breast cancer, she has the BRCA1 gene, she is 28 years old, recently went through chemo, 5 rounds, and has had a double mastectomy a week ago, during her first PET scan there was activity noted in her mediastinum lymph nodes, never biopsied, 2nd and 3rd PET scan no activity noted, my question is are those cells laying there waiting to come alive again and move on into another organ? She had recently had a child prior to be diagnosed and was breast feeding, her oncologist told us this may have just been inflammation from breast feeding and never was cancerous, what would you say? be concerned or not?