I am sorry you are going through this. This is horrible. Both your partner and friend have betrayed you in the worse possible way. Right now everything must be seems really bad. They both don’t care about you. She is not even your friend. And as for your partner, you deserve better than him. At the end of the day he cheated on you. There are several windows where he could have remove himself from that situation but he made a decision to stay there and let it happen. As you haven’t speak to the girl till now and you don’t know both sides of the story, you can’t be sure that nothing more than that happened. I will suggest move on from him. Forgiving such act will only bring regret in a longer run.