Sounds to me like he might be implying you're cheating too.
You need to sit her down and tell her the whole story BUT....this is not a competition, try and stick to the facts and nothing else. And try and make it about her, not about you (to avoid it feeling like she's being pulled apart by both of you)
- I know that you love SD and that you love me. I understand that this is an awful situation for you and for all of us. This is not something I ever wanted. I have not told you what I discovered and I'm sorry for that because I see now that you needed to understand what has been happening. I'm going to share some of that with you now. This is difficult to talk about, so bear with me.
- On [date] I discovered [information]. I confronted him about it and he said [blah blah]. I have since discovered [information] and now know that the affair with OW went on for [period of time] and was physical.
- He did want to reconcile, but has not shown any remorse in his actions towards me and I have been warned by other people that this is not the first time he has been unfaithful. Regardless of whether of not SD wants to reconcile, that decision is up to me and I have decided [course of action]
Now obviously your conversation won't be that cut and dried, but you see what I mean? Try and frame it from her point of view, just using facts, because too much from you could seem like manipulation, especially if she's already been primed by him.
Him: STBXWH 'The Arse' - Unremorseful passive aggressive blameshifter. Many EA's & at least 1 PA. Dday Aug 2012. Walked out April 2013. Demanded D, now stalling
Took a while, but I like the me I am, without him