I'm assuming that if you have the money to eat out every night it wouldn't matter if you were wasting money on buying groceries destined to be thrown away but it strikes me as being unethical to do so. The part I can't understand is why this hypothetical person wouldn't WANT to learn how to cook for both the control it gives them over their diet and for the sense of accomplishment that learning any new skill creates, unless they had some sort of learning difficulty, in which case their carer really ought to be looking after them to make sure they get a balanced diet.
I was still in kindergarten when I learned how to make my own scrambled eggs to serve on buttered toast where I'd stuck the bread in the slots, chosen the setting, and pressed the slider myself. It's true I wasn't allowed to do this without adult supervision and had to stand on a kitchen chair to reach the stove and the counter, but I did it myself and I was fit to burst with pride. If a 5 year old could manage a simple, hot meal why can't an adult?
Maybe your hypothetical adult is trying to run before they can even stand up? Can this adult open canned goods? Can they slice or dice an onion. Can they tell when potatoes and pasta are done? Maybe your adult needs to start with training wheels and brown some hamburger and add a jar of sauce while cooking spaghetti. Next time they might be confident enough to add some more vegetables, maybe some sliced peppers, between the meat and the sauce, the time after that maybe add some extra herbs, and then take the plunge and try making that sauce from scratch by following a recipe that they've carefully read through before shopping, read again before starting, and gave themselves plenty of time so as not to get stressed.
Here's a true story I recommend you NOT try at home. I used to work away for extended periods of time and that often meant sharing a house with a work crew. If you've ever worked these kinds of jobs you might know that people can get a little weird in these artificial constellations. One guy used to buy heaps of food, really nice food too, worth way more than the per diem, not touch it for a day or two and then throw it away. Soon thereafter someone in the know would rescue the food from the trash (seriously, don't do this) and cook it up into some sort of huge feast for everyone leaving the shared fridge overstuffed with leftovers which the original buyer would "sneak," but only when everyone else pretended not to notice. I got used to it, but it seemed like a very strange way to avoid cooking. The initial meal for everyone seemed like payment for someone taking responsibility for cooking it. Nobody ever officially discussed it with him and the process never started before the food hit the trash can. It's amazing what passes for "normal" when you spend too much time away from normal life. I wouldn't recommend this tactic because you're very unlikely to find out that your home harbours a trash can chef.