It depends on what type of acne she has .... I have oily skin prone to clogged pores due to the oil, but do not have much of the bacteria that causes "true" acne, according to my dermatologist. Differin worked fairly well for me, but the best prescription medicine I tried was definitely Tazorac. (Differin is prescription in the US but I think some girls get it from a Canadian website or something.)
The best OTC treatment I've found is Neutrogenia On-The-Spot treatment. It contains 2.5% benzoyl peroxide, which is enough to get rid of (most of) my blemishes without drying out my skin. All the other BP treatments I've found in the drugstores contain 10% BP, which dries out a lot of people's skin -- even my super-oily skin! (And none of my prescription medications ever dried out my skin!)
Just be careful if you're using BP -- it can bleach fabric, so I only use it at night and sleep in old t-shirts (or skimpy sexy things, lol) and use white pillowcases!