When a lumpectomy is done, if clear margins are not obtained (a clear border of non-cancerous tissue totally surrounding the cancerous tissue) then a mastectomy is required.
This is not known for certain until microscopic examination at biopsy.
Your friend should have been advised of this possibility prior to her surgery - I know I was.
It sounds likely that while some lymph nodes were excised as part of the lumpectomy, more were taken as part of the mastectomy.
While no cancerous cells were found in the original lymph biopsy, further biopsy found cancerous cells.
The treatment recommended will depend on full biopsy results as there are various forms of breast cancer which react differently to various treatments.