What are the long term effects for a celiac who doesn't follow the diet?

Cancer. Celiac disease is potentially a very serious disease. The immune reaction that occur from continued exposure to grains can place an individual at a very greatly risk of developing a certain type of cancer of the intestine. This is not a disease to take lightly.
Celiac. Celiac comes in many different manifestations. It just depends on how severely your body reacts to the gluten that you ingest. Over time you will learn if you can tolerate increasing amounts of gluten or you might find out that even the smallest amount causes symptoms. Much of this depends on how your body reacts. A diary is always very helpful!
Stimulus-response. Celiac is not a food allergy where you can sneak stuff in and just deal with any symptoms. EVERY time you consume the gluten, it stimulates antibody production that attacks your intestine and triggers inflammation. It may take years for the patient to realize they are in trouble, with intense intestinal damage or cancer. Living with this is hard but workable. Sometimes they don't realize the risk.