I would suggest buying a book like "opening up." Sorry, can't remember the author but its in our 'book and link recommendation' sticky if you want to find it. Then I would leave it out and read it. Tell her bits of it and how it resonates. Eventually tell the friend about it and see where that goes. Leave her with the idea and don't jump in until all of you have researched and become accustom to poly theory. They might have to catch up. If it feels right with this woman after all that, then address it. Anything before that might lead to issues. To me its better to be completely prepared. All of you be prepared.
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