I'm thinking that what people mean is that the moment someone agrees to at least go see a MC is not the time to lay out all your boundaries. That maybe that's what the MC is for.
It would seem we all agree that she should not have a relationship with OM but I don't know why he would have to say that in that very conversation. You men always seem to want to SETTLE everything instantaneously and on your terms. I agree that those should BE Tristan's terms, but I'm not sure I understand the point of laying out requirements BEFORE going to a MC. ???
Maybe she'll suprise you and set up this appt. She has time to think about some of these things herself before the appt. Tristan has time to think as well, and decide what he wants and how he wants to say it. Because, as a woman, I'm telling you, HOW you say it means EVERYTHING. Also discussing this within MC also makes her accountable to someone besides Tristan. Sometimes it's easier to have the accountability with someone you aren't emotionally connected to.
OTOH she may balk at really following through on this, and then you have your answer anyway.