You've got the 10% there, and you need the 5%.
10 is too strong and will wreck her skin. It's very very drying and can make the skin prone to burning in the sun, scaling with dehydration, and discolouration.
Look at acne.org for the regimen for back acne.
It's gentle persistence that will yield results.
They use cetaphil oily skin wash, a salacylic toner like Clinique toner number 3 and then the benzoyl peroxide 5 or even 2.5% and then a lightweight moisturiser. They also recommend an AHA so the wipes are good for clearing out the blocked pores.
No harsh 10%, no physical scrubbing, and use clean towels each time and change your pillow case every day too.
Be aware that benzoyl peroxide is a bleach and will bleach everything that comes into contact with it, including towels, pillows, sheets and her clothes. She needs white towels, tee shirts and pillowcases as she can bleach the colour out of everything she touches up against including upholstery if she's only wearing spaghetti tops.
She should see a dermatologist / aesthetician ideally and have the spots treated / extracted.
Also try taking her off all milk and cream. She can have cheese.
Also try going on the pill. Not necessarily an androgen blocker, but I found Cilest was great for my back acne.