Is this what I am reading?
- You and your husband are having trouble with your eldest child. To the point where you consider separating because you cannot agree how to best handle it?
- Husband suggests a threesome with a friend. You and the friend are concerned about pursuing the threesome at this time because of the family problems already going on?
If so, I agree. Not the best time. Sounds like the kid needs your time and energies most. Followed by the health of your marriage. Your plate already sounds full. Adding more people/relationships to the mix at this time doesn't sound like good timing.
You also do not sound enthusiastic about a threesome experiment at this time. So you could say "no, not willing at this time."
Could choose to spend some time with yourself figuring out what your own wants, needs, and limits are. Then could choose to talk with husband some more about what his wants, needs, and limits are. Is husband after casual sex? Swinging? Polyamory? What about you?
Because the couple does not seem really clear on what you both hope to gain from the experience or what it is you both are wanting from either the marriage relationship or the relationship with your friend. What's friend want?
What kind of open relationship model
are you and husband talking about?