The doctor told her that treatment has come a long way and survivability rates are high now
This is good advice. At this point, it sounds like you don’t even know the extent of the tumor. A significant proportion of breast cancers are resectable, which is curative, if it hasn’t spread beyond the primary tumor. But even if it has, contemporary management of cancer is pretty amazing.
The most common procedure is a lumpectomy and sentinel lymph node biopsy (to check for microscopic spread), followed by local radiation. They’ll also measure hormone receptors and other markers of disease progression. If those tests are positive, she may be given the option to take pills to eradicate any residual cancer/lessen the risk of recurrence. Only more widespread disease would require chemotherapy and/or immunotherapy - these are the areas where dramatic improvement have been made in recent years.
But really no need to get ahead of oneself. Find a breast surgeon and oncologist you trust, preferably at a multidisciplinary cancer center. Most importantly, do everything you can to support your wife. Cancer is always a dreaded diagnosis, and because of their association with gender and sexuality, cancer of the breasts is especially troubling to women. Even if she appears stoic, you wife needs you to lean on. Don’t focus on delays in the diagnosis, but channel your energy towards her treatment and recovery. Good luck.
ETA My wife had localized breast cancer diagnosed almost a decade ago. She had the tumor removed, followed by elective mastectomy of both breasts. She never needed chemo or radiation. It wasn’t easy at the time, but she remains disease-free to this day, with no evidence of cancer other than breast implants, which aren’t a terrible consolation prize.