Making hot food kills of a lot of the bacteria and viruses.

Great question. I don’t know. I would like to know. I think it too, and I don’t know why. My guess is hot food is a good treat on cold mornings and winter. Hot oatmeal is supposed to “stick to your ribs”, on a cold day. What ever that means.
Maybe because hot food costs more.
Some foods I won’t eat unless they are hot. Like steak or cheese.

Of course heat doesn’t add nutrition. Only enjoyment of the food.

Exactly @Birthday Boy!

I think it’s an old fashioned concept, being a “good hearty hot meal.”

I just ate some cold leftover roast chicken and some cold cranberry sauce and I’m all set. However, there are some that would feel that having cold chicken for dinner is a sad meal.

I think in the case of elderly people, a hot meal might help warm them up.

It sounds to me probably like people who have inaccurate ideas based on old expressions and ways of categorizing food. A “hot meal” or a “cooked meal” may have meant something of substance as opposed to a snack of less substance or nutritional variety, such as just eating some bread. Your friend may have built a misunderstanding of such expressions into her thinking and now like many adults her ego and comfortable thought habits have her unwilling to think or hear otherwise.

As @ragingloli wrote, enough heat can kill some pathogens. It also tends to reduce nutritional value a bit. Though of course some things are also a lot more edible when cooked.

My baked beans are as good cold as they are hot.
I also like cold pork chops, steak and fried chicken.
My grandson likes frozen corn dogs! Like, right out the box before it’s ever cooked.

I think it’s from the expression avoid hot meal. A hot meal was probably seen as either a luxury, or as more substantial than a cold meal. Especially in cold weather a hot meal can be comforting and satisfying. Hot meal for many I think stirs up a complete meal of an animal protein, a starch, and a veggie. That sounds more hearty than a cold sandwich. The nutrition might actually be the saw between the two, but the hot meal sounds more like a proper dinner.

There is no real reason that hit might be better than cold. Some people believe cold is better, like the raw diet.

I have never heard the expression “avoid hot meal”. I am not sure how it could be viewed as a “luxury.” Campfires have been a thing for hundreds of thousands, if not millions, of years.