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).