go read on www.acne.org
I buy their products. I like the products and have continued to use them for a few years now. From what the guy says on his website, he has a 'regimen' of wash, then benzoyle peroxide, then moisturizer. He says part of the key to getting the acne under control is to wait 10-15 minutes between each step, allowing the product to absorb.
Also, I think Mary Kay has a teen product. What would be nice about that, is generally you can return/exchange Mary Kay products if you don't like them.
If all else fails, go see the doctor and see what they say.
BTW - I had similar acne problems shortly after getting off of birth control. I'd never had acne down my back before or on my chest. It was bad. It took a while for the pill to wear off I guess, as the problem just went away for me. Perhaps your daugther has a hormone imbalance/issue or it's just part of growing up. Your dr would be better trained to tell you if that's the case.