Sorry you're going through this.
Part of what her doctors will do is monitor her at intervals even after her chemotherapy (and/or radiation) are finished, and depending on her cancer, she may well take medication for a longish ttrials. Some women take anti estrogen meds for years after chemo/ radiation for certain types of breast cancer, for example.
Depending on the type of cancer, location, etc, she may well need blood tests, MRIs, mammograms, pap smears, bone density scans, or biopsies at regular intervals for a good few years.
I don't think cancer patients ever get an unconditional all-clear.