A licorice salve would work great. The problem is you have to probably make it yourself because I don't know of a company that makes one. here is a recipe if you want to try and make it:
first you have to make licorice oil. Put dry licorice root in a glass jar with 1/2 cup oil (canola or olive). Make sure the licorice is covered with the oil. Secure the lid on the jar. Now get out your crock pot and fill it with water and put the jar in the water up to the level just below the lid (so water doesn't get in the jar). Cook on low heat for 2-3 days. Then strain the plant material out and reserve the oil.
To make the salve:
in a double boiler heat the licorice oil and 1 oz beeswax. Once the beeswax is melted pour the hot mixture into a sterile container. When it cools it should be a salve consistency. You can apply it too her feet throughout the day.
This is a small production and I don't expect you to have the time to do this, but I thought I would throw the suggestion out there because if you can't find anything else that works to heal her feet this slave could really help.
Good luck