Traditionally, is prosciutto never to be cooked?

'Never to be cooked'… nope. Cook it if you need it cooked.

Saltimbocca alla Romana I'd think to be traditional enough to refute this easily. Jamie Oliver's recipe. There are a million others, but the main ingredients are veal, sage & prosciutto… cooked.

For those questioning Jamie Oliver's credentials on this - Wikipedia lists the same major ingredients - https://en.wikipedia.org/wiki/Saltimbocca
as does this one - in Italian too - https://www.tavolartegusto.it/ricetta/saltimbocca-alla-romana-ricetta-originale/

The problem with saying the word "traditionally" in regard to Italian cooking is that traditions vary wildly from region to region (and often village to village). The person you were talking to is either absolutely correct or not at all accurate (depending entirely on the scope of the word traditionally in their statement).

According to my Italian grandmother, if you're making any kind of sauce, use pancetta or guanciale (depending on its purpose in the recipe) for best results. Prosciutto is "what you use if you can't get the good stuff." I even remember her occasionally apologizing for using prosciutto because our local grocer didn't always have pancetta in stock.

That being said, I have compared family recipes with friends where theirs specifically calls for prosciutto where my recipe says to make something else if you can't get guanciale.

I believe I can explain where your friend's belief comes from.

There are three main traditional cured pork products used in Italian cuisine: prosciutto, pancetta, and guanciale. Of these, (certified) prosciutto is the most expensive, and also the only one that is usually eaten without further cooking. As such, most Italians would use pancetta or guanciale for recipes where the pork is going to be cooked, just as an American would use supermarket ham or bacon and not 2-year-aged Virginia Ham for a soup.

Prosciutto also has less fat than pancetta or guanciale, and many recipes that involve cooking cured pork are depending on the rendered pork fat to then cook other ingredients.

Confusing this for Americans, most of what gets sold as "prosciutto" here isn't actually DOC prosciutto, but is in fact some kind of cheaper local ham. Making it, ironically, better as a cooking ingredient.

So it's not that it's bad to cook prosciutto; it's just expensive and sometimes inefficient.

I think the more complete and critical answer is from @FuzzyChef.

I think it captures the essence of the problem.

Some more details should be added regarding what's the situation here in Italy. Disclaimer: I'm an Italian living in Italy and I like good cuisine, but I'm not nearly a professional nor a gourmet*.

What you call "prosciutto" in USA here is called, more specifically, "prosciutto crudo", especially when one wants to make a difference between "prosciutto crudo" and "prosciutto cotto", which are fairly different products. Note that "cotto" means "cooked" in Italian, but, as I said, "prosciutto cotto" is not "prosciutto crudo" that has been cooked!

There is no single "DOC" prosciutto (crudo) in Italy. There are a couple of high valued, "DOC" types (very expensive): i.e. "prosciutto crudo di Parma" and "prosciutto crudo S. Daniele", which are somewhat similar products (to a non-gourmet).

But there are also a metric ton of quality types of "prosciutto" made outside the typical areas where "Parma" and "S.Daniele" are produced. The biggest difference is an higher percentage of salt employed during the seasoning. Often those kind of "prosciutto" are called "prosciutto crudo nostrano" ("nostrano" means "made in our area/region") and they cost less than "Parma" or "S.Daniele".

Prosciutto crudo is used in recipes where it ends up being cooked (e.g. Tortellini alla Bolognese, where it is used in the filling), but often you don't use the highly priced "Parma" or "S.Daniele" for that.

As for "pancetta" and "guanciale" they are lower price products here in Italy, because they are meant to be used for cooking, although you can definitely eat "pancetta" raw. It is a very fatty product and also very tasty, and a "panino con la pancetta" (sandwich with "pancetta") is very yummy (and a calorie bomb)!

Keep in mind that in Italy we have probably the most strict legislation against food fraud in the world. In particular, we have an entire branch of one of our national police forces (Carabinieri) who is devoted mainly to prevent and repress food frauds: they are the NAS (Nucleo Anti-Sofisticazioni).

This means that it is very difficult here to be sold "bad food", since an inspection from the NAS finding, say, a restaurant or a food shop selling expired food or (worse) bad food can cause the immediate closure of the business (until further investigations or trials), even if no-one of the customers felt sick!

All this to underline that what you eat in USA and it is sold as "pancetta" or "prosciutto" may not be what we in Italy exactly would expect. We have a huge problem of fake Italian products sold in every part of the world (even worse outside EU), especially where there are customers rich enough to pay a bonus for original Italian stuff (USA, Canada and Australia, for example).

Bottom line: if you buy traditional Italian food in the USA, be sure your shopkeeper sells you legit products, otherwise all bets are off whether or not the products are even remotely the same as the "traditional" ones.

Cooking with a high quality (and assumedly fairly expensive) prosciutto is like using fillet mignon for stew meat or sushi grade tuna for a fish stew. You are obliterating the great flavors and there's probably a much cheaper and better thing to use instead. Of course there are always exceptions and never say never but it's a good general rule.

What do you mean by traditional? The current de facto standard? What is the standard -- and whose standard? Is "traditional" what your grandparents were used to? What was in the cookbooks 50, 100, or 200 years ago?

Most of the time, I find that the word "traditional" is just gatekeeping. What they're really saying is "I don't like the way you did that, and I want to be right without a debate" or worse yet, "You're not one of us." It's bad form and people who do that should stop. It's fine to be interested in historical methods and preparations, such as what The Townsends does with 18th Century American (and British) cooking and what Glen and Friends Cooking does with old Canadian and US cookbooks, but that history should never be used to limit another cook's approach.

Prosciutto is just an ingredient. It should be used as you see fit and experimented with. People have always experimented with flavors and made replacements when their usual ingredient was scarce. That's how we get new foods! I love a nice bit of freshly sliced thin prosciutto, but I've also had it fried up crispy to complement a dish.

Now, for a Bolognese or "Sunday gravy", I'd probably reach for pancetta, guanciale, or even bacon, but that's mostly because I want the fat and the subtle flavors of prosciutto probably wouldn't stand up unless I used a lot of it. But for a lighter sauce made in a pan where I wanted a bit of cured pork, frying some chopped up prosciutto could be really nice. But then, is the prosciutto really part of the sauce? Are the capers and olives really part of the sauce of pasta alla puttanesca? Does it matter?

You can cook anything you want, but prosciutto (and Spanish jamón) are not their best when cooked.

The first reason is that they tend to be expensive, and they can be better enjoyed raw.

The second reason is that when you cook them, they don't become crispy like bacon, but hard and rubbery, and they become too salty as well.