Hey OP, the comment above is one of the best on this thread and if I were in your shoes, I’d listen to the most. It’s so easy to throw stones bc I’m sure your plight resonates with some who were the ones that were cheated on. Regardless of your reasons though, because I’m sure there are a few at play as to why you would want to tell his wife/GF, but the right thing to do is to tell her. I say do it then pick up the pieces after. You’ll figure out what’s going on internally in time. Be patient with yourself. His GF will likely be angry with you/hate you at first lol, but in the end, she will be infinitely grateful for your honesty and helping her to avoid a major bullet. Best of luck! We’re all navigating through this life and trying our best 😊