Q. I have a wonderful, beautiful, intelligent best friend who has had some difficulty in the dating department (busy career, etc.). Let’s call her Maggie. Several years ago, Maggie became friends with a co-worker, “Joe.’’ Maggie and Joe’s friendship turned into a sexual relationship and then a dating relationship, which continues to this day. Only problem: He is (and has been) married with a child. As far as Maggie and I are aware, Joe’s wife has no idea about this relationship. For several years now, Maggie and Joe have continued to see each other a few times a month and talk very frequently. While Maggie acknowledges that she is, essentially, a mistress, and knows that their affair is “wrong,’’ she is in love.

Maggie claims to be happy in the relationship and content with the fact that Joe will never leave his wife. She believes he can love two people at the same time. As a friend, I have been struggling with how to talk to Maggie about this relationship. On the one hand, I want her to be happy, I want to be supportive of her decisions and relationships, and I want her to feel like she can confide in me. On the other hand, I have a real moral problem with this affair and don’t think it is healthy for her in the long run (I worry she will get hurt and also think it prevents her from dating single men with whom she could share a full life). I do tell her my thoughts about the relationship, but worry that it is driving a wedge between us because she seems not to bring him up to me anymore.

How do I remain a good friend when I think my friend is doing a bad thing? Should I put my feelings aside and just support her? Should I encourage her to end the relationship? Do I have some moral obligation to tell his wife?Friend

A. Forget telling the wife. That’s one step too far at this point, and your focus should be Maggie.

Your best bet is to tell Maggie everything you told us. You want her to be happy, but you can’t condone her behavior. You want to be her close friend, but Joe is making it difficult for the two of you to communicate.

The point is, her decisions involving Joe don’t just affect Maggie, Joe, Joe’s wife, and his kid. They also affect every relationship in Maggie’s life — including her friendships. If she’s going to continue this affair, she has to understand that it isolates her in more ways than she might have imagined. It’s something to talk about when the issue is raised. See what she has to say about that.

Often in long friendships, we wind up watching someone we care about do something we think is bad — sometimes for a very long time. Some friendships survive, some don’t. Tell Maggie that you want to preserve yours, but that it doesn’t work if you have to lie to her. It could be that you wind up taking some space. It could be that you won’t be as close until she ends the relationship on her own. As long as you’re honest on your end, you’ve done your part.

Meredith

READERS RESPOND:

It sounds like you’ve made yourself pretty clear. There’s nothing you need to do at this point. If you find you can’t be a friend, then don’t pretend to be one.

WIZEN

I don’t think you are going to change Maggie’s mind with anything that you say, and I think at this point you have to decide if this is really a dealbreaker in the friendship you have with Maggie.

ASH

Sometimes, when you are a really, really good friend, you can smack them upside the head and tell them to look at what a mess they are making of their life and someone else’s. Be that good friend, letter writer.

THENURSE

The definition of insanity is doing the same thing over and over and expecting different results. The letter writer is hoping that conversation #534 about this immoral affair will finally get Maggie to see the light. Not happening.

SHIPYARDLOVER

The best solution is probably for you to seduce Joe.

MCDIMMERSON

Column and comments are edited and reprinted from boston.com/loveletters. Send letters to meredith.goldstein@globe.com.