In the midst of an anxiety attack or nervous breakdown-there is only one route and that is comfort until they are safe and securely right-minded again.
I say that AS a poly person who is completely supportive of open relationships. Because I also have anxiety issues and I had a serious nervous breakdown or mental break this last winter.
There is no reasoning with someone who is in that state-they can't even reason with themselves and its unfair to try and enforce it.
It's also unreasonable to make any sort of future plans, agreements or commitments (including marriage) with someone whilst they are in that state.
The only right answer is to stop, assess, comfort and heal.
THEN one can discuss the future, commitments and potential agreements.
"Love As Thou Wilt"