Tinea versicolor. Get some over-the counter clotrimazole 1% and have her apply it twice a day for 2 weeks. This is a non-catchy fungus that grows in our follicles and appears on the chest and back of young people. When it comes back, she can reapply the cream. If the cream doesn't work, a doctor whould have a look.
Best.
Dr. Rockoff
I'm not a dermatologist, but I've seen something similar to what you describe and it sounds fungal to me. The area between or under the breasts, especially in large breasted women or those who wear certain underwire type bras, can collect and hold onto moisture especially in warmer weather, which is a breeding ground for fungus. It's not dangerous or life threatening. Try an over the counter antifungal cream for 10 days-2 weeks and see if that clears it up. Also keeping the skin clean and dry should help prevent it.