To what end?
Will a lawsuit be worth the exacerbated negative energy that will surround his life while he pursues this?
In this case, I think the time, cost and energy that would go into a civil case like this is not worth it. Is money what he would be after? Is clearing his name what he would be after? Will either of these things be accomplished and will the time investment of his life be worth it?
I think he should have loftier goals and he should move in a different positive and forward direction. I think if there is any smudge on his name, it can be cleared by continuing to do good works.
JMHO of course