The risk of developing smoking-related diseases for a non-smoker is significantly lower compared to a smoker. Non-smokers are at a much lower risk of developing lung cancer, heart disease, stroke, and other smoking-related illnesses. However, it is important to note that non-smokers can still be exposed to secondhand smoke, which can also increase their risk of developing certain health conditions. Overall, the risk of developing smoking-related diseases for a non-smoker is relatively low compared to a smoker.