Woman you've known for years, have worked together and got on with (but not a "friend") has a baby with live in boyfriend. You find out boyfriend is cheating on woman, very stupidly bravely having a full on relationship with other woman while playing happy families with woman you know.
Other woman works with woman and knows exactly what she's doing (isn't being lied to about the situation, she simply doesn't care about what she's doing)
Baby is 1 week old.
Would you tell woman?