Stuck in the Middle

Dear Mod and Maude,

I have two friends that are a couple. I used to be better friends with T, the guy, but now I seem to be closer to the girl, B. B has revealed to me that she's been cheating on T for awhile now, with multiple guys. She's recently called and left me a message saying that she told T she was hanging out with me when she was actually going to see another guy. I didn't get a chance to say whether or not I agreed to this arrangement. I know she's planning on leaving him eventually, and his world will be wrecked - I really feel like she needs to do it now rather than later. She talks about her cheating in such a cavalier way and I'm having trouble not saying anything judgmental. Should I say something to B, and if so, what?

Thanks,
Inadvertently Aiding and Abetting

Dear IA&A,
Maude: Life is simply too short not to be judgmental! Let’s look at what you’re gaining by keeping your mouth shut. Hmm…
a) A friendship with a girl whom you do not respect and do not trust (if she’s lying to him, what will keep her from lying to you?), who – I promise this will be one of her misguided steps – will "blame" you for something at some point.
b) The very solid chance that you will get a phone call from the guy in crisis as he has somehow contracted an STD. He will just not know how that could happened… you get the picture.
c) A follow-up call from your brokenhearted friend who has just discovered he’s been cheated on and has so many questions.
d) And, a very guilty conscious throughout.
I think that you will be forced at some point to part ways with this dishonest and immature person: this girl (I applaud you for not calling her a woman) who takes no responsibility for her BF or her friend's well being. So--yes, you most definitely should say something to B. That would be "goodbye". Just in case you are not up to that type of absoluteness, you also have the option of telling B that she has 24 hours to come clean with T or you will spill the beans.
Courage.
Mod: Ew. What a scorching stew of emotional ethics. First things first, no matter how forcefully you want to draw the line about B’s cheating, she had no right to say she was with you when she wasn’t. That’s classic 13-year-old saying you’re at a sleepover when you’re meeting boys in the park behavior: when you get caught, everyone is in trouble, even the poor girl who didn’t know she was having a sleepover. You need to let B know that this behavior was not ok with you and can’t happen again. Then, since you’ve launched into the conversation, you must tell her how you feel about the situation (obvs, prepare this speech ahead of time so that she doesn’t pull you in with her wily ways). In my mind (which, as usual, is a bit less dramatic than Maude’s), your line of speech could go something like this:
1. Tell her that you disagree with her choices and that they are making it hard to be her friend (it’s possible that no one has actually come out and said this)
2. Ask her what she’s planning to do. If she doesn’t know, or doesn’t plan to tell your friend, let her know that you disagree with this.
3. If she still doesn’t commit to telling him, tell her that you are not going to be able to stay friends with her (because she’s hurting one of your friends) and you will eagerly await her call when she’s come clean.
4. Finally, and this is only something you can decide, determine whether you should tell T. Whose friendship are you most invested in keeping in the long run? If B breaks up with him and doesn’t tell him about the cheating, is it better for him not to know? If he finds out about the cheating and asks you if you knew about it, what will you say?
Make a decision, stick with it, and buy yourself a $10 margarita for having to put up with other people’s obnoxious folly.