As my wife has informed recently, it's obvious to her, that even though I've always been faithful to her in our 27 year marriage....that I am definately poly.....I can't argue with that. I am the guy in "I love you, man"...the guy with only girl friends. Obviously this other woman can't handle the poly lifestyle, thinks it's sick or is just possessive over "her" man. None of these bodes well for you, your guy or the poly lifestyle. Possessiveness cannot exist in poly. Even though you and your guy may want and be into the poly situation and give your permission, she won't necessarily want to deal with it. She wants your guy to her own and that's that.
Ovbiously it takes a certain mix to make the poly lifestyle work. That's the key. It's tough under even the best circumstances. How tough is it to find the perfect mate? Then another perfect mate, yet or another?....very tough