Both of my best friends are dating. In this case, lets call them Romeo and Juliet. They like each other a lot, always held each others hands, blah, blah, blah. It was all absolutely brilliant! But I wouldn't be writing to you if it all stayed peachy. One day Romeo decides to be slightly dirt-bag-esque and starts cheating on my friend. I didn't breathe a word of this to Juliet, but went straight to Romeo and told him gently and sensibly that what he's doing is wrong. But he won't stop.
Should I tell Juliet about this situation? I know how hard this would be but this girl is like my twin, we have literally grown up together. It would kill me to see her hurt. So how am I supposed to alert her to this situation in a calm way?
Welllllll, for starters, you might want to avoid describing young Romeo's behavior as "slightly dirt-bag-esque"—and start treating it like the gold-plated, grade-A display of asshattery that it is. I mean, seriously? You told him "gently and sensibly" that what he's doing is wrong? How about telling him not-so-gently that he's a jackass for cheating, that he's a dumbass for putting you in the middle of it, and that it'll be his ass if he doesn't stop it right now?
And actually, that's not a rhetorical question; it should also be more or less your next move, and your go-to strategy should you ever find yourself in a similar situation again. Urging him to do the right thing establishes you firmly on the side of human decency while also establishing you outside the bounds of the mess he's making. (Which, of course, is where you should have been all along, and you'll really be doing your juicebox friend a favor by teaching him an important lesson about the utter self-sabotaging stupidity of telling your girlfriend's bestie that you're cheating on her.) And in cases like this, where we're talking not about a single instance of poor judgment but rather an active and ongoing betrayal, it is important to condemn the cheating even as you distance yourself from it—and with something stronger than the gentle/sensible moral guidance you'd use on a three year-old who cheated at Candyland. Anything less suggests that you approve of his assy behavior.
Which brings me to this: the cheating is gross, and the fact that he made you a party to it, knowing it would put you in the middle of a seriously uncomfy moral bind, is grosser... and to be honest, Sparkler, I'm the teensiest bit surprised that you're not more ticked off about it. You wouldn't, by any chance, happen to have feelings for this guy yourself, would you? Because if so, it's important that you admit your biases, check your self-interest, and make your next move only in service of doing what's best for your friend.
And as for that: your friend needs to know that she's not Romeo's only Juliet (especially since depending on the nature of these relationships, her health could be at risk in addition to her heart). And since ideally she'd hear this from him, your best bet is the ultimatum: he tells her, or you're going to.
And if he doesn't? Then you do—as gently as possible, with her best interests at heart, and without couching her boyfriend's betrayal in diminishing language like "slightly dirt-bag-esque." Because these might be your two best friends, but one of them deserves much better.
Have you ever had to give bad news to a good friend? Tell us about it in the comments! And to get advice from Auntie, email her at firstname.lastname@example.org.