One of the biggest reasons why people struggle to lose weight is lack of discipline. Even one occasional burger can have a significant impact because it breaks the habit of healthy eating. If this is a behaviour that he shows often, it is not conducive to his goal of being healthy.
A good solution would be to find alternatives that he could prepare easily and would not take him a lot of time. Alternatively, he could still eat out but try to find something that is more balanced and nutritional.