Mark, I found this:
"Expression of CgA has been documented in epithelial cells of normal mammary gland as well as in breast cancers, and elevation of serum CgA has been detected in patients with breast cancer. ... serum CgA was not an accurate marker to identify or exclude the rare neuroendocrine differentiation of breast cancer. We therefore conclude that CgA is not useful as a serum marker in breast cancer." Read more here: https://pubmed.ncbi.nlm.nih.gov/11820723/

Fear of recurrence is a cancer patient's (and family members') constant companion. This recent spike can be due to a number of things. To know for sure if it is cancer-related, additional tests would need to be done. Has her oncologist recommended any imaging studies?