BFIQ, let’s start with what you shouldn’t do, which is talk to him. That’s overstepping a major boundary. Talking to him would be full-on meddling. All you can focus on is your relationship with your best friend.
There’s a common misconception out there that people are supposed to be there for their friends, no matter what. That’s mostly true, but it sort of depends on the “what.” The “what” in this case is a married guy.
Your friend is asking you to lie. She’s asking you to play along with something you believe is wrong. She’s asking for too much.
Don’t make this about what’s best for her. Don’t spend your time predicting her miserable future with this man. Make this about what’s best for you. Tell her that you don’t want to lie anymore, and that every time you hear about him, you feel as though you’re betraying someone else’s marriage. Tell her that you love her and want the best for her. Tell her that it’s painful to watch your best friend commit to someone who can’t love her full time. Explain that hearing about this man has become a source of stress for you.
After you’re done with the love and flattery, state your boundaries. Tell her that if her relationship comes up in public, you won’t lie on her (and his) behalf. Tell her you won’t play along with the boyfriend talk and that you’d prefer not to hear it. Make sure she knows that these boundaries are meant to help — and that you’re trying to save the friendship. You want to be a good friend, but she should want to return the favor. The fact that she’s asking you to lie makes me think she hasn’t considered how any of this makes you feel.
I have to be honest — she might push you away. But this is becoming toxic for you. All you can do is tell her the truth, love her, and take care of yourself.
Readers? Is this reader taking someone else’s relationship too personally? Is this friend asking for too much? How can the letter writer keep her sanity and her friend? How awesome are our new, shiny letters? Share, please.

– Meredith