I'd recommend that your wife wears shoes at the store that are more comfortable and supportive.
There are some dress shoes that accomplish that for women. Depending on her age, it would be worth an evaluation for osteoarthritis. The treatments can include tylenol, (acetaminophen) nsaids (like aleve), steroid injections, and use of a cain. Regardless, the knee pain is likely improved with physical therapy.
Would you like to video or text chat with me?