Dear S. W., Milk allergies are generally an allergic reaction to the protein components in milk. In adults the incidence is even lower.
Chances are they are experiencing a mild degree of lactose intolerance.
Lactose is the sugar found naturally in milk and milk products. Many people make the mistake of avoiding all milk and dairy products from their diet, and by doing so they are depriving themselves of major and important nutrients their body needs. A recent study found that most people with lactose intolerance can consume up to 2 cups of milk or dairy products per day without experiencing symptoms. For example, my husband, who was also diagnosed as lactose intolerant, can eat one dip of ice cream or use one half cup of milk on cereal without getting the gas and bloating symptoms. People who give up dairy products tend to consume far less calcium than they need, putting themselves at risk for the bone thinning disease osteoporosis.