I really feel for you and your son - it's never fun to be in a situation where there is a downside to any path you choose. But if your son benefitted from the experience, then you likely made the right decision to push forward.
The only thing I might suggest considering is that children, especially highly intelligent children, are often quite perceptive and understand when things are wrong. And when they don't have enough facts, the information they conjure to fill the void of understanding can often be more damaging than a simplified version of the truth.
I remember being in a similar situation with my daughter, and I think I told her something like, "You know, it's really kind of sad, but sometimes grownups haven't learned all the lessons they should have learned in elementary school about being polite and kind and considerate of others. And that's what you witnessed. It doesn't look very good on a grown up, does it? And, you know, whenever I get after you for maybe being mean or impatient with a friend, now you know why. I want you to be able to be the kind of grownup that isn't unkind to other people."
Your son doesn't need to know the source of the mother's bad behavior, or that his being there had anything to do with it. But he did witness bad behavior, and having that not only acknowledged but put into perspective may actually help him feel more secure, not less.
Again, just a thought - take it or leave it as it fits your own style and knowledge of your own child.
