Sandi always says, don't cover up her affair. If she says it's amicable in front of you, then out her. If you do nothing, then you are implicitly condoning it and staying a doormat.
My STBXWW told someone on a group chat that we're trying to make things work, I immediately responded "so you think that spending your bday with another man and coming home at 10am the next day is trying to make your M work?" Safe to say, she's not tried that stunt again.
Just cos things are going right, doesn't mean that they were always wrong.
