I had moderate acne as a late teen, but as itchy as hell - ended up with the doctor, ages with antibiotics - did nothing; roaccutane, better but spent 7 months on it contrary to the 4 months theory and being blood tests regularly for potential liver failure; dianette - much better, but then you gain weight and get a bit moodier - oh joy!
I have been told by a doctor friend that Yasmin is the modern version of dianette, and can really help.
Your DD really needs to see the doctor and a dermatologist, as different people have different reactions to things. Also spots in different places indicate different things, and they are different types of acne - eg for me it is round the jaw (apparently normal with PCOS) and indicates hormone problems.
I have adult acne (started back up 2 years post baby due to PCOS) - and have found that it flares up with cow's milk (apparently its the hormones in the milk), wheat and sugar. Dark chocolate in small amounts is fine, high sugar stuff isn't. Also Nelson's do a homeopathic skin wash etc sold in Boots which is quite calming on the skin.
As a teenager I was recommended to drink aloe vera juice (but made me gag), and there are loads of things that you might read about - its all about finding your triggers. Mum told me recently that teaching out in the South Pacific, she never once saw a teenage islander with spots - basically they had no cow's milk, wheat (used corn flour) or sugar in their diet, and ate lots of oily fish. So there could well be something in the diet, even though junk / fast food companies will obviously say that there is no link - if you have the money you can find a research project to do whatever you want it to do.
Good luck, and take her to the doctor, the sooner you crack it, the less problem you have with scarring and acne forming under the scars .......