Sounds easy but we're deep in "draw the rest of the fucking owl" territory here.

For some people the threshold for healthy eating habits is low. I have to actively try to gain weight and I can handle slight hunger well so for me very little would have to change on a diet.

That person I mentioned, on the other hand, has to pre-plan their nutrition for a week down to the macronutrients of each single meal, just like a bodybuilder. A planning mistake will most likely result in binge eating. And yes, they would eat half a loaf of full-grain bread at night if nothing else was there.

So for someone like them advice like "have some self control and don't buy junk food" is about as helpful as "have you tried not being poor". Getting their endocrine system and gut microbiome to accept anything below maintenance calories is a long and nontrivial journey.