Have you talked to him about how much she sucks and how crappy he's become? I mean, not in those words, but it's something I'd try. Get your friends together and have an intervention. How long have they been together? It can be really hard to get out of a relationship that's lasted a long time. You have to show him the light, man.

Occasionally we have to deal with the fact that someone we love (platatonic or otherwise) has changed. Everyone changes, most notably as a result of outside influence through those around them. Ever introduced someone you knew to a band, only for them to utterly fall in love with said band? It's like that, only more pronounced. This effect only becomes heightened when introduced by a loved one. Naturally we all want to impress the ones we love, and equally we spend inordinate amounts of time with them, so, maybe this guy has simply changed during his time with this girl due to her influence, and maybe he doesn't see it as all bad. Look at it from his perspective.

as true as your cliams might be, you only have the power to tell him how you feel and not tell him what you think he should do.....poeple dont make decisions based on friends (who they have kissed~*the advice you give will seem biased) he will most likely get out of the relationship cuz it wont stop sucking. and if he has kissed you that seems to me that he has already, though maybe only subconsciencely, started to end it. dont worry, I doubt if she really depresses him that much and he is already got his lips on other girls, that it will last long.
bottom line you dont have the power to end it, but you can be a good friend and tell him what you see, and that you wish him to be happy, then stick with the friendship through his hard times, like good friends do!
good luck!

if he loves her and wants to marry her-- then he wouldnt be lip-locking with you in the first place. espeically if he didnt want to tell her.

ix-nay.

He'll figure it out for himself. It might even take years. If you but in, he will most likely just get mad at you for trashing his lady love. Sit back and let him do the leg work himself.

word to that.

Talk to him.. try to open his eyes about her. She's not good news.

If he kissed you he's probably not that fond of her after all. Tell him you think she's a huge bitch.

let it go, he will see eventually and you will just end up looking like you are trying to win him over. been there done that, it doesnt work. find a better guy with no girlie and makeout with him, or hell just makeout with me!