Your mother could be experiencing some type of inflammation. It could possibly be inflammatory breast cancer. Since it is under her breast, in the fold maybe not. Does it look like an orange peal, bumpy and reddish. Very few doctors are familiar with this condition.
If she had been trying to cure this rash for several months and has not succeeded, I would be concerned. If it is indeed a rash she could be deficient in Omega 3 oils. It could be any nutrient deficiency. It is a toxic release of some kind.
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I would not use talc. Talc has been linked to ovarian cancer and has been found to induce cancer in rodents.
Best of health to you and your mother.