Unless you actually KNOW something is actually going on and aren't just basing your concerns over gossip that you've heard I would stay out of it for sure. At the moment you've just heard through someone else that she's seeing someone else. That may or may not be true. Going to your friend and telling him anything at this point is just spreading gossip. I agree that you'd be better off going to her and telling her what you've heard in a non-confrontational manner. Things like this can get really blown out of proportion really quickly.
Everything will be ok in the end. If it's not ok it's not the end.