All good advice
I totally agree with what seems to be the overall sentiments of the group here that at minimum a level of friendship is really important for any long term stability.
I think any "poly" relationship that involves one of the group really NOT being poly is dangerous water to cross and if there's someone kind of "out of the loop" then it's the seeds of disaster. We end up back at that all important "C" word - communication. If you can't all communicate openly & honestly the hole is in the boat so to speak. Just a question of how long before she sinks.
A possible additional note to these situations that seem to crop up a lot....
Whenever we have found situations of a possible poly/mono connection, we've learned to be VERY careful - if not avoid them entirely - unless we have a long, sit down and talk session with all involved INCLUDING especially the mono person.
It's all to common in these type of situations for that one mono person to be "just going along to keep peace", hoping it's a phase that their poly-inclined partner is going through etc, and that internally they are roiling and in extreme anguish. We don't want to be a party to that unless there is some extreme extenuating circumstance we discover that seems to justify it. We've never found that circumstance to date !
So advice here is - be careful, put up your antennas, go slow and be honest about what you detect - even if it exposes a reality different than the dream.