I was also the OW when I was 19, he was 29, married with two boys. I had no idea what his DW would go through or feel, I just didn't understand because the only long-term relationship that I had, I finished because I just wasn't feeling 'it' (whatever 'it' is supposed to be...).
He left her after about a week and moved back in with his parents. We saw each other twice in that time and then he moved back home. We worked together but I made sure that once he had made his decision I kept completely away from him.
About two years later he contacted me again, I was working for a new company in a different city and he'd found how who I worked for and called me. He said that he was still with his DW but was really miserable. I told him that if he was looking to get out, if he had any balls at all he would leave his DW first and then find someone else and that the someone else was definitely not going to be me! We have a mutual friend (she is one of my best friends and Godmother to DD but she worked in his team so knew him well) and about a year ago she said that he'd contacted her on Facebook to say hi, that he was still with his wife and they seemed really happy.
BUT I had no idea how his DW must have felt when I was 19, I only started to understand when I got married to DH. Neither of us have had an affair so I still can't understand to an extent but I understand what love is now and I'm scared of the potential to lose that. The experience is a million times worse I imagine.
So in that long post my message is, I wish my parents had said something to me. Neither of them did. They even spoke pleasantly to him when he came to pick me up one day. They knew the situation and they said nothing, even though 5 years before my Dad had an affair.
I swear that I would use my own experience to make sure my DD is fully aware of what can happen and that it is morally wrong on so many levels.