First, my dictionary has ser/estar(Spain) viudo, but I think it's even more complicated than a dialect issue. Here's a relevant discussion: https://forum.wordreference.com/threads/to-be-a-widow.596749/
Note that the discussion extends to several other relationship-like words such as soltero (but like viudo, these words are to my mind not relationship words, but rather civil/personal status words, which is why I think ser is often correct with them).
With "viudo" and other words that express civil status, "estar" is the most common verb used in Peninsular Spanish. You are right by saying that "ser" is valid, however, when we use "ser" it will generally be in a more formal context, for example when someone is requiring information from you to fill in a document or in a formal interview. For example: "- ¿Es usted soltero, casado o viudo? - Soy viudo."
In Latin America the use of "ser" may be more common than in Spain, though.
Because it is a basic lesson, we chose, as I mentioned before, the most common use in Peninsular Spanish.
Gracias y un saludo
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