Why do you feel you need to tell her that he doesn't like her? If her crush is a secret between the two of you, and she's in no danger of losing face, then consider keeping your knowledge to yourself while encouraging your friend's interest in other guys. If she's making a fool of herself in front of him, or if he's about to tell her -- in no uncertain terms -- to take a hike, then you'd be doing her a favor by telling her, gently, that the guy's not interested. You can't avoid hurting her feelings. But you'd be sparing her some pain by cluing her in. (Wouldn't you want to know if the shoe were on the other foot?)