Don't worry about how you found out. The fact is, you found out. Just tell her "I know all about you and _____, including the financial help, and it needs to stop. It's inappropriate and while I know that I myself have been a real hot mess at times in the past, the fact is I've expressed my regret about that and I'm working on it. But I'm not going to remain in a marriage where my wife is carrying on that kind of relationship with another guy."
Do NOT, under any circumstances, reveal the source of your intel. And keep DBing.
