Not knowing you, or your dad, or the situation at all makes it really hard to give specific advice. Do it somewhere private and don’t set it up with the phrase “I have something to tell you” or anything like that. That just creates anxiety and no one will ever be able to say that for good news again without triggering him.

He might already suspect or even know. Or it could be a total shock. So private is best because his reaction could be intense.

Beyond that, buddy I just don’t know. Depends a lot on the people. If you think it’ll be a confrontation, don’t tell him she’s cheating, just present the evidence. I saw this, I heard that, I found this. Let him draw his own conclusions.

No one is genuinely authentic. Maybe a few people who are incapable of doing otherwise due to issues.

Sometimes I’m not even authentic to myself. I can be different people from one day to the next. Generally it’s an attempt to be better followed by reversion to mean, but sometimes not.