First of all, let's talk about the definition of diet. Diet is defined by Merriam-Webster as "food and drink regularly provided or consumed; habitual nourishment." So, no, we're not talking about a "diet" here that deals with no carbs, low carbs, cabbage, cookies, etc. Diet here is referred to as daily sustenance. So whatever this person is consuming on a regular basis isn't helping them reach their fitness goals. You can spend several hours a day at the gym working your absolute hardest, but if you go home and slam fast food and pizza all day, you're being counterproductive. Plain and simple, you will not see optimal results if you are overeating and making poor food choices, no matter how much time you invest in the gym. Make sense?
Let's flip this debate on its head. Say a worst-case scenario happens. You head into the gym one day and during a warm-up set on a compound lift you feel a pop and drop to the floor. Yikes, an injury. Hey, we're human and it happens. So suddenly you go from a gym rat who can't miss a day without feeling depressed to someone who is laid up on the couch and can barely bend over to put shoes on. Working out is obviously not an option, and even when it becomes one, it needs to happen slowly and progressively. This is the perfect example of a time when you realize the importance of diet and how big of an impact it has on your physique and leanness. Tailor your diet correctly and you won't balloon up like you probably feared you would. Body fat percentages will stay in check even though your physical activity is limited.
In the perfect world, a balance of diet and exercise is the key to physique perfection. While you can search "abs are made in the kitchen not the gym" and find a plethora of articles about the "percentage split" of diet to working out for the perfect body, there really is no exact science. Everybody responds differently to different approaches. Some people only need three days of heavy lifting and minimal cardio to feel and look their best. Others need five or six days and a moderate amount of cardio.
The thing is, until you experiment with various caloric intakes/macronutrient splits, high-intensity versus low-intensity cardio, heavy lifting versus high-volume lifting, etc., there is really no good way to know what works best for you. If you were faced with having to make a decision of only diet or exercise, diet would be the best to keep your physique in check. Of course, in the long run, exercise is obviously very important as well, not just for aesthetic purposes but for a slew of health benefits as well.
At the end of the day, the most important thing is to practice moderation. This not only is a good rule for diet and exercise, but for all areas of life. Too much of anything is, well, too much. Too much exercise has adverse effects (overtraining, etc.) Too much eating leads to weight gain and emotional issues (self-disappointment, etc.). Through some trial and error, you can find a balance that works for you. Keep your health in mind and at the forefront of your daily life and you’ll be in good shape.
