1) One negative effect of Luis not exercising when he's on a diet is that Luis's body is most likely not used to the change of food consumed and can lead to energy loss and/or vomiting. If Luis does not exercise to strengthen his body and become more prepared for change, Luis may get sick.
2) Mixing exercise with a healthy diet is a much more effective way to decrease in weight than depending on calorie or food restriction alone. Exercise can prevent or even reverse the effects of specific diseases. Exercise lowers blood pressure and cholesterol, which potentially can prevent a heart attack. Having both defenses is a great thing for your body.