It is your duty as a good friend to tell someone you care about how you truly feel. Hate your friend's new significant other? Tell them, tactfully, that you think they deserve better. Make it clear that this new squeeze does not treat them the way they deserve to be treated. Sure, you're putting your nose where it doesn't belong, but the alternative scenario is simply not acceptable. Be sure to frame your criticism in the most positive light possible, and show them you are only thinking of them.