Dear Dr. Wes & Kendra:

I have a friend who is dating a guy who cheats on her. I found this out because his roommate is my boyfriend’s friend and it’s basically going around the friend group and beyond. The only person who doesn’t know is my friend. We’re not super-close but I do care about what happens to her. But I don’t know if I should tell her and if I should, how to go about it.

Kendra: Keeping secrets within a friend group is like telling passengers on an airplane not to stand up as soon as they land; no one will listen and you still end up with a congested mess. And although you’re involuntarily stuck in the middle seat with no way out, as a friend, you have to take some action. I’m writing this column in the middle seat on a flight to visit my sister, so excuse the metaphor.

Romantic comedies have taught me two things: No one has to endure security in movies and people will almost always shoot the messenger who tells them their significant other is a cheater. However, the message is far better coming from you than from a stranger.

Because you’re not extremely close to your friend, I recommend first confronting the cheating boyfriend. Give him an ultimatum and a deadline: Either he tells her the truth or you will.

Assuming he doesn’t come through, tell your friend what you know and nothing more. For example, if you didn’t actually see the act of cheating but your boyfriend did, make that clear. When the rumor mill starts churning, there can often be a convoluted story. Your friend definitely does not need to know that everyone but her knows. Her closer friends may have been more concerned with losing her as a friend and didn’t tell, but don’t play the blame game.

While your friend may be upset, in the long run she’ll be happy you were the one in the middle seat because you were honest with her.

Wes: Boys aren’t going to like what I have to say about this, especially if they cheat on their girlfriends, but I’m increasingly convinced that above all else, girls need to stick together. I don’t care if their friends or not, girls share a sisterhood, and they should follow a code that transcends dating relationships. So yes, I’d tell her unless there is some compelling reason not to.

In assessing that, consider the ethic of beneficence — giving to someone in a way that is actually helpful to them. In other words, ask yourself if your disclosure will do any good for your friend. In this case, doing good probably means they either break up or do something pretty dramatic to end the cheating.

If on the other hand, your friend has already been around this bend a time or two with this guy and they’re still together, there’s a good chance she’ll overlook his cheating or more likely, she’ll break up then get back together with him after he’s confessed, been assigned his penitence and absolved. If that’s your friend’s usual position — and it’s pretty common these days — you may not want to risk the drama.

A second possible exception puts you in an even more unenviable seat of judgment. If you have good cause to believe that the boyfriend did this only once and there was some mitigating circumstance (e.g., intoxication), and if you believe he’s unlikely to do it again, it may do more harm than good to tell your friend. I’ll admit this is controversial, but the general thinking on infidelity is that it should only be disclosed when it will do a relationship good or help to end it.

As readers of this column are aware, I’m no fan of cheating. What teens and young adults practice now they grow into later. Cheating and tolerating cheating do not predispose anyone to a great romantic future. So evaluate the situation carefully along the lines that Kendra and I describe and take the most ethical, beneficent stance. And for everyone else, better to end a relationship honestly than to stomp on the heart of someone who trusts you and cares about you.

–Wes Crenshaw, Ph.D., ABPP, is author of “I Always Want to Be Where I’m Not: Successful Living with ADD & ADHD.” Learn about his practice Family Psychological Services at dr-wes.com. Kendra Schwartz is a Lawrence High School senior. Send your confidential 200-word question on adolescence and parenting to ask@dr-wes.com. Double Take opinions and advice are not a substitute for psychological services.