You are correct to be concerned about your friend's diet consisting of only milkshakes. It is important and would be beneficial to your friend to see a physician to have basic blood work drawn. In addition, she can be referred to a nutritionist
who can help educate her on the importance of a balanced diet, and simple ways to eat a balanced diet while listening to her reasons for maintaining a diet of milkshakes.
The types of ingredients described, including chocolate sweets and milk, are high in fat and high in sugar. Therefore these can lead to weight gain. Over time, this type of diet in addition to overweight can lead to problems such as elevated blood sugars, even leading to diabetes mellitus, which is an elevated blood sugar level, where the body is no longer able to lower the sugar with insulin made by the pancreas to a normal level. Diabetes has many complications, including on the heart, kidneys, eyes, nerves and other areas.
Many deficiencies could result in a sustained diet only consisting of milkshakes. While milk contains calcium and can contain vitamin D, it lacks many other nutrients. For example, meats and fish contain iron and B12, is essential for building new red blood cells which carry oxygen. Vegetables also contain iron, as well as folic acid, and fruits contain vitamin A, D, C, and E.
You may want to discuss these issues with your friend, though, again, she should see her doctor