When someone sleeps on the same side for years, what happens physically to their shoulder, hip, and neck?

Potential tendonitis. Those that side-sleep have a higher incidence of shoulder and hip tendobursitis. This incidence can increase with harder mattresses, obesity and smoking. Consequently, when treating hip or shoulder tendobursitis correcting certain habits can assist resolution, including reducing side-sleeping, smoking cessation, and weight loss. Pillow top or plush top mattresses may also reduce these problems.
No effect. When some sleep on either side off there body , during sleep if they have usally they turn around, that is natural reflex. The same thing when sleep in bad position and the hand get numbe you wake up and shake to relief it.