The correct way to handle it is to use the word "former" instead. "Former French President" parses correctly. "Ex-" binds tightly to the word in front of it be

The correct way to handle it is to use the word "former" instead. "Former French President" parses correctly. "Ex-" binds tightly to the word in front of it because of the dash, but "former" allows "French President" to be the noun phrase being modified.