You have no responsibility and no loyalty to her, only to yourself - it is not your place to speak with her, tell her or explain to her, that is his if he so chooses, he has ended it - presumably in my opinion because this night has shown him his feelings for her are not all that they should be. I think he has been as honest as he needs to be - no-one owns another person and she does not "own him". The past is the past and you need to focus on yourself and how you feel, not try to second guess her feelings, they are not your issue.
DHs ex actually thinks I am my poor blameless friend and that he was with me for years, because she once picked something up from their house - that however is her tarring everyone with the same brush because she used to bring her bit on the side into their home when DH was there. No amount of explaining that I wasn't her would wash. But that was all years ago.
DH too "hoped" something might happen when I had no clue, in hindsight, I think that was him sensing something that I didn't and not him manipulating a situation.
After the initial, how can we do this, we have known each other for ever, we didn't see this coming, we can't do this conversation, we just realised, it as right for us.
We have been through some times that would have ripped another couple apart (not through either of our fault sorry to sound mysterious), and we are still as together and as strong as ever, that long standing friendship stood us in good stead because when life feel apart, we had a lot of shared history even before "coupledom".
My only regret is DH and I didn't get together a long time before we did.