I think approaching this as a discussion rather than blurting out "I need to tell you I hate your gf" will be ultimately more productive and safeguard against ruining your friendship. If he understands you're looking out for his back but that you two don't see eye to eye, you'll probably be able to keep the relationship. If you calling her a raving, soulless lunatic who's turned him into a similar looney... maybe not.

Depends how long they've been dating. If it's been years, you may ave missed the boat, unless the girlfriend is actually a bad person. If it's early, just tell him straight up what's on your mind, but if he chooses her over you, he'll never forget what you said, so really think about if it's worth it.

Hard as it is, you just tell him. Be straight, not mean or blaming, just honest and from the heart. He may not agree, but he'll know you're only thinking of him when you speak your mind.

That's a rough one. I'm going through it right now with a friend as well. I ask myself, what are the possible outcomes? and none of them look good =/