Be prepared to ask for a lot of forgiveness from her. He's been cheating with you, which is a bad place to start poly. But very possible, lots of people have done so. Still, a core of poly is trust. .... and you and he have a lot to do to rebuild that.
Also be prepared for her to make him choose between you and him. Given the guilt he's likely to feel, he may very well cut you out of his life.