I know it sounds incredible. They lived with us for three months. She obviously wasn't bringing men over to our house. We would have known that. That started when they moved into their own apartment. So, yes, she hid her true colors and for a long time. In a forum about divorce, separation, and annulment, that doesn't happen?
Here's the deal though... because you're an outsider to their relationship...
Has your son ever previously lived on his own? What is the nature of his disability? Is it REMOTELY possible that while she seemed willing and eager to take on the burden of providing care... she didn't REALLY know what that entailed because they'd never been on their own? Is it possible that your son isn't quite the same as a HUSBAND as he is a SON, and since you're only hearing his side of things, you don't have the whole picture? It sounds like he's invading her privacy, having expectations that perhaps he didn't before (his anger at her "leaving him alone"), etc.
It's very very rare that a marriage falls apart based on the actions of only one of the people involved.
THAT is why I suggest counseling for your son. So that he can learn to correctly allocate responsibility, and take some on himself.