asking for a substitution in a restaurant

I personally wouldn't expect substitutions with a prix fixe menu unless I had an allergy to one of the offerings.

You can ask but be prepared for them to say, No. And if they agree, fine. Might also depend on the substitution. Peas for corn might work. Lobster instead of hamburger, probably not.

As Frank noted, "you can ask ..."
Places with a set menu might just refer you to the a la carte section where you can order exactly what you want. And pay for it as priced. Assuming they're cooking it that day.

Let us know how this works out,

I have only been to Italy twice.. a lot less then some other europeon countries and UK, so I am not very experienced with Italy , but, my feeling is , you can ask.. but you will usually not be granted a substitution .. it would be considered a ballsy thing to ask in some places.. especially when ordering the prix fixe .

I agree it is a far more common practice in North America.. but its not as common or acceptable in Europe.. but that would not stop me from asking if it seemed like a reasonable t

I think you're correct.

I wouldn't want to be dining with a person who was asking for substitutions on a price-fixed menu. If she doesn't like the combo listed, then order items separately and be willing to pay the cost.

I don't often see a prix fixe meal on a menu in Germany, but when I do, they usually fit together. I've never had an inclination to want a substitution, so I don't know what they would do if I did ask for a substitution. However, I think these specials are usually something they have prepared a lot of that day, at least the main course, so I wouldn't expect them to substitute that. You might have luck with something like a different vegetable or a salad, or maybe even späztle for rice.

I remember years ago I went into a restaurant in a small mountain town (Downieville, Frank) in Colorado and they had a special shown on the board. When I got the menu, I added up the items on the special, and it was less for ala carte than the special. When I ordered the item ala carte, the waitress said, "that's the special". "I know," I said, "but it's cheaper this way". She gave me a strange look - like you weren't supposed to figure that out.

Common, no.
Acceptable, only if you are asking for something of lesser value, such as something offered on a less expensive fixed menu. But don't then try to negotiate a lower price. It's your choice (really your sister's) choice to opt for a less expensive item.
When working in Paris we used to substitute an espresso for dessert at the end of a meal. I once substituted a less expensive fish, but that's in forty years of living and traveling in France, which shows you how uncommon it is.

Thank you everyone. I'll relay your replies to my sister.