A boundary would be K telling his W that if she CHOOSES to bring OM around the children then he will file for divorce and sole custody. Establishing consequences is just as important as the boundary itself. BUT, even his attny told him that such a boundary would be IMPOSSIBLE so in fact, it is NOT a boundary at all. It is merely a request and his W could choose to honor it or not. She chose not.
There seems to be an awful lot of confusion on what boundaries are, how to set and establish them and how to follow up with consequences should that boundary be crossed.
Because Kevin does not have a viable consequences (especially one that doesn't stem from the need to control his W and what she does and who she does it with) a boundary cannot exist.