Common causes of metallic tastes in the mouth are some medications, iodine supplements, kidney disease, heavy metal exposure, or arsenic.
Check side effects of any medications she is on. And look for exposure sources of metals or arsenic, such as if you are on well water or if she smokes. And check the kidneys, especially if she is diabetic. This increases the risk of kidney disease, and the drug metformin can also cause a metallic taste.
Other possibilities are bleeding in the mouth and sinus infections. Does she have gum disease causing bleeding? Any signs of a sinus infection?