Pruritus is a very common problem.Usually it is a symptom of underlying disease or a skin problem.But very often no cause or disease may be identified.
You have not said if your fiancee has any skin rashes along with pruritus .How long has she been suffering from this problem?To be eczema,there should be some kind of skin rash.Severe pruritus may also be due to urticaria,in which case there are hives.
If pruritis is severe at night and there are skin rashes in the finger webs,scabies has to be considered.
Pruritus may also be due to dry skin.If there are no skin rashes,the systemic causes of pruritus such as jaundice,diabetes have to be ruled out.
For proper diagnosis and evaluation,I would advice a dermatologist consultation.