"She is his second wife."
Translation:C'est sa seconde femme.
No, not "anything", there are rules.
In French, "c'est" (sing.) and "ce sont" (plural) are used in a large variety of expressions, when a pronoun (it, she, he, they) is subject of verb "être" and followed by a nominal group, ie: article (+ adjective) + noun. - it is + noun => c'est + article + noun - she is + noun => c'est + article + noun - he is + noun => c'est + article + noun - they are + noun => ce sont + article + noun
- note that these rules are also valid for possessive or demonstrative adjectives.
I agree, when he/she/it is followed by a definite or demonstrative modifier + noun, both translations should be accepted:
- she is a loyal wife = c'est une épouse/femme fidèle
- she is the best at swimming = elle est la meilleure en natation OR c'est la meilleure en natation
- she is his second wife = elle est sa seconde épouse/femme OR c'est sa seconde épouse/femme
"une mariée" is a bride, so the word is valid only on the wedding day.
If you need a synonym for "sa femme", you can use "son épouse".
Note that "son épouse" uses "son", even though "épouse" is feminine, because of the vowel sound conflict between "sa" and any next word starting with a vowel sound.