really sums up the problem.
Your best friend isn't a football, and you're not playing capture the flag with Lorena. Treat it like some sort of competition and you're guaranteed to lose. For whatever reason he's hung up on her and there's not a damn thing you can do about it. Which, I will freely admit, sucks, but there you go.
Having said that, stop playing the damn game with her. From what you've described she's getting off on it as much as she is twisting your friend around her finger. So... stop playing. Be nice, be polite, and when she gets involved leave. And make it (politely!) clear you're going because of her.
Sitting in the cafeteria and they come over? Say hi, and ask her politely to go sit somewhere else. If she doesn't, get your stuff together and leave. At a football game or whatever and they stop by? Be nice and ask she goes somewhere else. If not, get up and move. If you don't want to go, ask them both to go sit somewhere else. Do that especially if she starts enjoying forcing you out of places. If you're out with him doing stuff and she calls, get him to bring you home first and if he won't then bug out yourself and call someone else for a ride or walk home or whatever.
At some point he will clue in that something's up (likely the first time you ask them both to go away) and get pissy about it. You will get, in one form or another, a "you're making me choose!" accusation, if he doesn't just go missing entirely. When it comes, be clear that you're not making him choose. You don't like Lorena. You're not obligated to. Since you don't, you're not going to spend time around her. That's your choice, not Justin's. You aren't forcing him to choose -- he can do whatever the hell he wants. You're the one choosing, and you choose to not spend time with someone you don't like.
And yeah, you may lose him over this, but I'd bet not. If he's as good a friend as you say he used to be, at some point he'll clue in to the fact that he's been yanking you around to cater to his girlfriend, and you've chosen to not be yanked around any more. He'll either accomodate you, or he won't. If he does, great, and if not then it's better to get it over with now. It's possible you may have to decide whether you're willing to make some accomodations of your own for him.
Basically you need to draw your boundaries and make it clear you want him to respect them. If you can manage this without heat (which will be very tough, but try as hard as you possibly can to be calm and not get upset whenever this happens) you've got a damn good chance of this all working out relatively well, and if it doesn't work out you'll at least have tried and will ultimately be spared the hassle of having to deal with her.
Getting into pissing matches with a friend's SO always dooms you to misery. That's what you're doing with Lorena. So... put it back in your pants, there's no way to 'win' this one, and playing just gets your shoes wet.
-Dan
This was excellent. Couldn't have said it better.
Sharon