Well, this is obviously no longer poly, if it ever really was. I think you have been doing the right thing all long, not being pushy or trying to interfere in their relationship, but just being there to support her when she needed you. I think you both seem level headed .
Just carry on doing what you are doing. Be thankful for what you have for now and give her plenty of space, and I think things will turn out well. I should not worry much about her going back to him , I very much doubt that will happen but if they manage to remain friends then you should be happy for her.
It looks like she is in love with you, she said so to him as well, just give her time and space.
Nice guys don't always finish last eh? Good luck
"The secret of life, though, is to fall seven times but to get up eight times"