"their (singular)" in French

“Leur”agrees in number with the noun that follows, not with the number of people we’re talking to. It is the same in both masculine and feminine forms.

e.g:

Marie asks Michel : “What’s the name of Sophie and Gilles’ son?” (Comment s’appelle le fils de Sophie et Gilles ?)

Michel replies: “Their son is called Julien.” (Leur fils s’appelle Julien.) because son (fils) is a singular word in this context.