Saddening is the -ing form of sadden, so it still means “to make someone feel sad,” but you meet it in two ways. As part of the verb, it is the action happening now: the news is saddening her. More often it works like a describing word: a saddening story is a story that makes people feel sad. It names the feeling the thing causes, not a person who is already sad (that person is just sad). It is a quiet, rather formal word, closer to writing than to chat.