Here are the drugstore products that are helpful for back acne. If it doesn't clear in a month or two though, please see a dermatologist. Using a salicylic acid based acne wash twice a day or at least once helps. Get a back brush with a handle and apply the wash, leave it on for a minute in the shower and then wash it off with the brush again. 

You can use the benzoyl peroxide acne gels in the drugstore on the back. They sometimes stain clothes so apply it at night and then have her wear an old tee shirt to bed. Both of these products may take 8 weeks to work.  Again, if you don't see a difference after 8 weeks, she'll need a visit with her pediatrician or a dermatologist. For more information see my in depth article on Acne. Dr. I