Personally, I would feel somewhat uneasy about that master/submissive SM thing that they've got going. And about the fact that she defines herself as poly and yet "she thought she would never have another love besides him, but I came along". Since when is she poly? Add to which that you can't have a "monogamous" relationship with her if she's having a relationship with him.
What you can have is a case of her cheating on him with you. And that brings me back to my opening uneasiness. If she's masochistic, can you be sure that she's not using you as grounds for "Punish me! I've been a bad little girl."
And if he's into domination and sadism, what happens if he goes after you? How big is this guy anyway?