Hi Tigs!
The causes of acne and dermatitis from a chinese medicine point of view can be rather varied. It depends on several specific things that would need to be accessed by actually seeing your friend.
However.... there will most likely be some common areas:
Damp - this is likely to be getting trapped under the skin, causing the susceptibility to have a skin condition in the first place. Damp-based skin conditions tend to be characterised by pus-like skin eruptions, such as acne or some types of eczema, but it is usually involved to some extent, even if the condition is relatively dry.
Damp is generated in the body by the digestive system and is "stored" in the Lungs, which are also very closely connected with the skin (think asthma/ezcema connection). Therefore a weakness in the Spleen and Stomach (digestive system) and/or the Lungs may be pre-disposing your friend to forming Damp.
Many dietry factors are also involved in producing Damp in the body, such as how and when we eat, what we eat, and our state of mind when we eat. Also, studying weakens the Spleen as does worrying. (There are other factors too...!)
Treatment with acupuncture (or herbs etc) will tend to focus on clearing Damp from the body and strengthening the digestive system so that it doesn't produce it so much in the first place. The Lungs may also require strengthening. Also, dietry advice would be given specific to your friend to help her reduce the kinds of foods that tend to form Damp, and help her eat in a way that nourishes the digestive system.
Next up is Heat. This is often involved in skin conditions, for example where there is redness, swelling, the sensation of heat. Heat can be caused by a great many factors. Blockages in energy due to the suppression (or lack of expression) of emotions, for example anger can create Heat. Blockages in energy can be caused by many other factors. Heat can also be generated by a relative lack of Yin or cooling energy. Other symptoms of this may be restlessness, trouble sleeping, sweaty palms...
Treatment would focus on clearing Heat, and treating the underlying causes of Heat, for example emotions counselling, and acupoints that help the free flow of emotional expression, or the strengthening of Yin energy.
I have found that the Small Intestine is often involved in skin conditions. In Chinese medicine, one of the functions of the Small Intestine is to "separate the pure from the impure" (it has this function physiologically as well, but it wouldn't be put like that!) In Chinese medicine, this function extends to waste outside of the bowel: dermatitis can be viewed as a disfunction of the Small Intestine in that there is a disfunction in the secretion of toxins into the skin.
Other indications that a person has a weakness in their Small Intestine may be a tendency to hoard unnecessary things, having unusual priorities, being relatively obsessive about organising and arranging things.
Treatment would focus on strengthening the function of the Small Intestine.
There will be several other factors, and some practitioners would list different things in priority to these. The actual diagnosis would come from a consultation with your friend and examination of her pulse and tongue. Without these bits of information, everything else is just speculation at this point, as I'm sure you apprecitate anyway
As always in chinese medicine, we try to treat at the most underlying level, as this is both simplest, and has the most broad ranging benefits. We may find that all the factors mentioned above may be treated by just one or two acupoints that affect the patient at a core level.
Hope that is of some interest! As always, this is just my perspective on it, and I can't speak for any other chinese medical practitoner!
L&L
Aragorn
(Italics indic