My mom doesn't have large feet, but she does have a lot of issues with shoes. Due to an accident, she has rods in her legs, and as she's gotten older her bones have shrunk but her rods did not; this causes her a good deal of pain sometimes as she is "lopsided" in her gait.
She kept buying shoes that felt comfortable in the store, but after wearing them for a bit, they ended up being very uncomfortable for her. I kept telling her to go to a running store so they could fit her, and she kept putting it off. Finally, when she came to visit me the last time I took her myself, and they fitted her with a pair of New Balance (she tried several).
Though the shoes cost 3x as much as other shoes she's bought, they're the first shoes she's worn in years that remained comfortable after wearing them around for awhile. Instead of needing to replace them right away due to discomfort, she's saving money by spending more in the first place!
Definitely get her to a running store (not the Nike store, or New Balance, Dick's, Famous Footwear, etc., but a REAL running store with multiple shoe options; do a Google search for ones near you) and have her fitted!