It's possible that she's experiencing polymorphous light eruption, or PMLE. Another possible diagnosis is Actinic prurigo. Both are conditions related to exposure to sunlight. Does she get itching or spots on her lips?
Either way, the only way to get a diagnosis is to visit a dermatologist. There are tests that doctors can perform:
* Photo-testing - a small area of skin is exposed to measured amounts of ultraviolet light . If skin symptoms appear after this exposure, the test confirms that the skin eruption is sun-related.
* A skin biopsy- a small piece of skin is removed and examined in a laboratory
* Blood tests to rule out systemic lupus erythematosus (SLE or lupus) or discoid systemic lupus erythematosus