The fitness expert/trainer will need to modify workouts according to a person's age. For example, the target heart rates are lower for a sixty-year-old than they are for a twenty-five-year-old. Knowing what those target heart rates are will in turn determine workout duration and intensity. Likewise, workouts will need to be adjusted for a woman who is seven months pregnant. Having her train for a 5-kilometer race is likely an error of judgement on your part. The extra weight that the woman is carrying is extremely hard on her back and knee joints. Running will only make those things worse, in many cases. The focus should be on finding ways to keep muscle tone and adjustments in diet in order to maximize the right kind of nutrients needed for the mother and developing baby.
As for the diabetic patient, you should already know that type 2 diabetes means that proper diet and exercise can be used to control the disease. You should start by learning how severe the person's illness is and what steps he or she currently has in place to control the disease. That is your base starting point. From there, offer your client small dietary changes that need to start happening. Do not start with telling your client everything that he or she is doing wrong. That will seem overwhelming and condescending. You are trying to build this person's confidence that he or she can manage the disease through diet and exercise.
As for the exercise, factor in what this client needs to do. If he or she is overweight, you need to emphasize cardio in order to begin burning more calories. Stress that the type of exercise is not crucial. Walking, running, cycling, swimming, and other forms all function as great cardio workouts, so if your client hates one of them, don't force him/her to do that one. Motivation is going to be key, so you need to find an exercise that the person is going to want to keep doing.