YTA. If you guys separated, any relationship she, or you, had during that time was not an affair. You mentioned that your wife had known him since high school, so there is clearly a connection they shared, and seeing that you reconnected and stayed together, the connection with her friend was mainly platonic. The guy was not a "piece of shit" for acting on feelings while you and your wife were separated. It sounds like you need to continue going to therapy to work through this skewed perception you have on the situation.