You have to believe that if she does decide to leave him it will not be about you. It will be her inability to cope with loving a polyamorous person. Trust me, if she is wired the same way I am she is not taking this lightly and is definitely struggling especially at first...by not running already she has shown great strength, just as he has shown great strength in risking losing her by following his nature. It doesn't mean she doesn't love him, it just means the pain/pleasure balance is too tilted to the pain side. There is no fault in this when two totally different natures emerge...it simply is nature.
Compersion is wonderful. I have complete compersion for what Redpepper and her husband have. I am a huge advocate of long term partners getting to know each other and become comfortable that everyone is secure. Honestly, I will be extremely challenged by the introduction of another man into her life and only at that time will be able to assess my own pleasure/pain balance. I have gone down the road of guessing and it completely takes away from the present.
Nature take us along paths chosen from deep inside us…We're all only human...there is nothing unnatural in responding to how our nature dictates.
Nothing that happens will truly be the result of your presence in his life, it will be the result of human nature, his and hers.