Fiancee has a rash under her breast and she wont let me touch it. How can she get this rash to go away?

Shingles? If it's under only one breast, and she won't let you touch it because it hurts, it's probably shingles. She'll need some wet dressings and a prescription for valacyclovir or famciclovir. Other possibilities include yeast or bacterial infections, contact dermatitis, and herpes simplex.