You mentioned that your friend is since 2 months on duolin, for treating her Allergic asthma. Any form of asthma, will not cause this kind of nipple or breast discharge. This maybe due to an hormonal imbalance or due to an infection which must be detected and treated according to the diagnosis and not symptomatically. You can consult with the following tests as advised so that timely action can be taken before it gets worse. 
Get serum prolactin and CBC blood tests done.
If prolactin is high get her to consult an endocrinologist, or else consult a Gynecologist for treatment and further management.

Prolactin
Prolactin is a hormone produced by your pituitary gland which sits at the bottom of the brain. It causes breasts to grow and develop and causes milk to be made after a baby is born. 
Normally, both men and women have small amounts of prolactin in their blood. During pregnancy, prolactin levels go up. High prolactin levels trigger the body to make milk for breastfeeding. For normal women who are not pregnant, prolactin helps regulate the menstrual cycle. In men, it affects sperm production.



Hyperprolactinemia

Hyperprolactinemia is a condition of too much prolactin in the blood of women who are not pregnant and in men. Hyperprolactinemia is relatively common in women. About a third of women in their childbearing years with irregular periods but normal ovaries suffer from this disease. 
When this happens, a woman's breasts may start producing milk outside of pregnancy or face trouble getting pregnant. Ninety percent of women with galactorrhea also have hyperprolactinemia. High prolactin levels interfere with the normal production of other hormones, such as estrogen and progesterone. This can change or stop ovulation (the release of an egg from the ovary). It can also lead to irregular or missed periods. Some women have high prolactin levels without any symptoms.

In men, high prolactin levels can cause galactorrhea, impotence (inability to have an erection during sex), reduced desire for sex, and infertility. A man with untreated hyperprolactinemia may make less sperm or no sperm at all.

Common causes of Hyperprolactinemia

Some common causes are:

No cause is found in about a third of all cases of hyperprolactinemia.


Treatment

The treatment depends on the cause. If no cause is found or you have a tumor of the pituitary gland, the usual treatment is medicine. Hypothyroidism is treated with thyroid replacement medicine, which should also make prolactin levels return to normal. 
If your regular medicine is the reason for your high prolactin levels, check with your doctor who would work with you to find a different medicine or add one to help your prolactin levels go down.

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