The neighbour may be "trustworthy" as a source but i have to wonder about the type of person she is to even bring this up in idle chit-chat with someone who is an acquaintance.
Surely anyone with an ounce of integrity would be able to see that there is two sides to a story and that it was a very sad chapter in a couples lives and it doesn't need regurgitated by others' who have no place talking about it. I know all of my neighbours yet none of them have a clue about what goes on in my lifyoue and vice-versa.
You are right
I think that good people do bad things sometimes, and whilst having an affair isn't exactly honourable, there is no such thing as black and white situations. Think of all the good she does in her work as a therapist... bad or selfish or cold people simply could not do that job. And chances are she's had her own therapy to process what happened in her marriage.