A few years ago I worked with a woman and her adult son.
They were nice. They attended all the out of work functions including family fun days etc. They both socialised with some people in the office and their children in their homes.
He was the jailed for abusing children. It turned our that he had already been jailed once and neither he or her told people he was awaiting trial again.
I get he was her son. But she put her colleagues and friends children at risk by helping him hide this.
OP, you do realise that in neither of your situations, you didnt actually find something out. Your second example, shows that it wasnt definite that your colleague sent that letter. It could have been someone who randomly, hated the woman, the letter was sent about for reasons that had nothing to do with work?
The second incident, that woman could have been lying. Or wrong. Maybe he left her. But maybe it wasnt for your friend. I know my dps ex wife will tell people we had an affair. Which would be difficult because we didnt meet until both of us had left our spouses. She acknowledged the date he left, when she kicked him out and told him it was over, in the divorce papers. He relocated back to his home town (no small children, just one adult son who lives abroad). I met home 3 months later and we didnt date for a while after that. As soon as she found out, she started contacting everyone in his family telling them how he had left after meeting me. It was actually his family that introduced us, so they all knew this wasnt true.
If my friend believed someone they dont know saying that I had done something awful, I wouldnt consider them a good friend. Also you left your friend vulnerable to someone stalking them.
Besides which you cant steal someone partner. That alone says woman easnt seeing the situation as it was. At worst he cheated and left her. Stalking the woman's fb and contacting people who know her at all, isn't normal behaviour.