Unless I'm missing something, I thought the OP had said that the new relationship was very much ongoing.
The problem is catinhell that unless you've been witness over the years to X telling Y that she no longer loves him, or indeed witness to every nuance in their marriage (impossible obviously) you can't in all honesty say that the marriage is as X is describing now. It's fair enough to have an overall impression of the marriage based on your conversations and interventions over the years, but who really knows why your IL felt the way she did so frequently? Perhaps she was permanently frustrated about the changes Y kept promising that never materialised.
I've often thought, for example, that what can look like a marriage between a laid-back person and a control freak is actually the reverse and that the laid-back person is the real controller in the marriage, but in a very passive way that evades detection from outsiders...
Having said all of that, if X had got complacent and used to a script of being the one who was loved more, it often happens that an affair or sudden emotional detachment from their spouse shakes them out of their complacency and causes a re-think.
Y isn't being fair at all on the third party, because rebound or overlap relationships are blighted by the sort of recriminations he quite justifiably fears. After a long relationship, it is far more healthy to be on his own, rather than using another person to propel him out of a relationship he might have lacked the courage to leave. That person has feelings and emotions too and it would be unfair to use her as a pawn in a marital game.
In her shoes though, I would want to be absolutely certain that no lies were being told, including those by omission. I wouldn't want to be party to any deceit, whatever I was being told about a cold, unloving wife who has cried wolf once too often.
The other thing is although I understand your need for privacy and your hesitation in posting too much, I'm curious about how you can be so definite that this is a new relationship. I got the impression that this was relatively short-lived, which makes the "soul mate" guff seem even more suspicious. Infatuation? Maybe.