I'm not a doctor or medical professional, so I can only give you suggestions. I don't know why there would be a workup (or additional testing) unless: 1) there was an abnormality on the mammogram, or 2) the mammogram was normal, but there is a palpable lump or thickening that is of concern (mammograms miss up to 30 percent of breast cancers, particularly in younger women). Additional testing may mean anything to additional compression view mammography, sonogram, or even a biopsy. It depends on how far they need to proceed in order to definitively rule out (or diagnose, as the case may be) breast cancer.
I hope all turns out well for your wife. Please let us know, and we'll be hoping for the best for her.
"There's a difference between a philosophy and a bumper sticker." --Charles Schulz