"but what has she to lose by telling the truth? It could not hurt my son more."
The very painful fact may be that she may already have told your son the truth. She may have told him on many occasions and in very clear terms.
I have a friend who split up with her partner, for reasons that were fully explained to him. I know she explained because on more than one occasion I was with her when she was trying to explain it (yet again, 5 months, 6 months, one year on, tearfully, on the phone) but he really did have a desperately difficult time processing what had happened. Five years later he persuaded a new girlfriend to track my friend down to ask what had happened and why had she left. It was heartbreaking - the new girlfriend thought that a quick, though painful conversation, would release him from his torment. He just needed "his answer."
Five years on - after countless attempts to explain directly - and he was still struggling to process it and believed, quite truly, that he did not know why my friend had acted as she did. I am not saying this is definitely the same case for your son, but there is a risk that it could be and I don't know that a mother, however close and loving would be in any position to know for certain. The ex's family believed his version wholly - they were all bright, educated reasonable people, as was he, but it wasn't the reality.
Please encourage your son to find relationship counselling - find a safe environment, away from friends and family to assess what has happened to him and how to move forward. This isn't a one off, it's twice now. It really would be terrible to get caught in the mental trap my friend's ex was in. All his family, by his account, saw my friend as the bad woman who had left him for no reason because that is what he told them. And yet none of that was true.