Well I am a MIL, so I don't have a natural bias against 'em, and my late MIL was a total sweetie. I agree that people of both sexes do leave out of the blue sometimes and that maybe the poor fellow has indeed done nothing wrong - it happens. However I also believe strongly that whatever this woman has done she is not accountable to his mother. Frankly, had my MIL been alive when I split with her son, I would not have well received a letter saying I should explain it to him because "honesty could help him understand". You can believe me or not as you wish, but nobody could have been more honest or anxious to make herself understood than I was. However the only "explanation" XH would have understood was "I made a mistake so I'm coming back to you", preferably with a side order of "I am so sorry to have tried to leave that I will let you walk all over me to the end of my days". Anything else, he just wasn't hearing.
Funnily enough he too veered between saying I was leaving for another man, and saying he had no idea what my problem was (when I patently wasn't with another man, or he was talking to someone who knew me better than that). And you could have sworn, listening to him, that he believed every word.
So: no, you don't know, and unfortunately you probably never will. You may be harassing a woman who was at the end of her tether but is too nice to dump the facts on you, or pleading with a heartless harpy who couldn't care less whether her ex gets closure on their relationship because she's moved on. The truth, as usual, is probably somewhere in between.
Re counselling, he may benefit from some if only to get to the bottom of why he has twice picked a woman who would do this to him.
Oh, and I call bullshit on the "can't take ADs" claim too. I bet he hasn't sat down and had a good talk with his occupational health adviser and/or GP.