I had one about 8 years ago, on my right heel. I tried just about everything to get rid of it besides going to a podiatrist, but those things are seriously stubborn, and 8 months later, I'd had no success. Because it had gone that long without being cured, I wasn't able to walk properly on my heel because it was painful, the wart had gotten so large and deep. I eventually had to go to a podiatrist to get it cut out. He told me that in 99.9% of cases, over-the-counter plantar wart therapies don't work, and patients end up having to have them excised surgically anyway, just because of the nature of the wart.
I'd say best to just take her to the podiatrist and get it over with, rather than have it go on and on and on to the point where walking is painful. I had developed a limp and it took me a good 3-6 months to walk normally again after it was removed.