First rule out fungal infection
Thanks for the additional details. The distribution of the rash beneath the breasts(in the folds), as you say, throws another possibility of a fungal infection. Fungus thrives in moist body folds like groins, sub-mammary folds, axilla etc.
The best way to approach this is to get a KOH scrapping from the rash to look for fungal infection.
If it comes out to be +ve, it is definitely fungal. Negative result rules out fungus and leads us back to the previous diagnosis
If the rash in the sub-mammary folds does turn out to be fungal, you can ask her to use a topical antifungal
cream like clotrimazole
twice daily for 4-6 weeks. An oral antifungal
can also be added for faster results.
One the infection is cured, an Antifungal dusting powder can be used regularly to keep the area dry and unfavourable for fungal growth.