ETA: I didn’t read responses before I posted. I see now your father has offered excuses for his behavior. This isn’t uncommon. They’re also not to be believed.. Back to my original response...)
You say your mother’s not in a good place, emotionally. I wonder: could this be, in last, because she has cognitive dissonance between what her husband is pretending to be, and what she senses (as we BSs often do, long before we have concrete info)?
Your mother’s mental health is certainly a concern, but I would feel very betrayed if someone I loved and trusted made a paternalistic decision NOT to give me vital information needed to make vital life decisions. It’s not up to others to decide with what I am able to cope.
That said, you can identify things that might help your mother cope, like SI.
FWIW, while I absolutely LOST it when I learned of my ex’s (last) infidelity, my overall mental health improved ENORMOUSLY. I lost it, short term, because I’d been so terribly manipulated/lied to/gaslighted. I didn’t trust my own perceptions. And that continued after dday; a counselor specializing in trauma was worth her weight in gold. Secrets would simply have made things SO much worse, when I found out (as people almost always do).
I know your intentions are good. But you’re not aware of all that makes up your parents’ relationship. It’s theirs, and your mother deserves all of the information necessary to make an informed decision about her relationship with your father.
If you hadn’t already done sleuthing, etc., I might counsel differently. I might say, tell your dad you’re concerned, that you hope you’re mistaken & that he’ll do the right thing but, regardless; your mom needs to know what’s going on so that she can make informed decisions about her life.
But you’ve gathered evidence and know he’s having an affair. That he’s doing it so overtly suggests a few things: that it’s not his first time; that he’s using YOU as a proxy to inform your mother (not sure I’d assume this role, at least not right off the bat), and —worst—that he currently respects YOU as little as he respects your mother. (This is likely a function of the idiocy that infidelity wreaks, and may well resolve as he pulls himself out of infidelity.)
In your shoes, I might use the last thing as a jumping -off point with him. It is unconscionable that he’s chosen to conduct an affair in your presence, and I’d let him know this. I’d make it clear that, no, he’s NOT so clever that he’s concealed it. He’s cheating, you know it, and you’re FURIOUS (and hurt and heartbroken for your mom and for your family—whatever you might feel) that he’s felt entitled to do so.
Think about the arrogance required for a man to conduct an affair in the presence of a daughter who also loves her mother!
I would NOT continue to keep his secret. Your mother’s mental health is NOT protected by guarding your father’s secrets. The secrets—which she may well suspect—may be contributing to her struggles; the truth, while unspeakably painful, may be a genuine relief. I know it was, for me.
I don’t think I’d bother to engage in any sort of negotiation or bargaining (“you tell her by X date, or I will”) with your father. He’s already demonstrating profound disrespect for you, by involving you both in his sordid affair AND marital issues. My father did the latter, and it was terribly damaging. There’s nothing to be gained from bargaining.
Identify some ways you can support your mom. Maybe find a good IC who specializes in trauma (skip the MC; that’s for her to decide, and depending on how long she’s been lied to/gaslighted, she may not be ready for a LONG time—that’s one last thing on the topic I WOULD share with your father, if he tries to pull you in. This is NOT about her, her drinking, or their marriage. It’s about HIS selfish decision to seek ego kibble from another vulnerable woman. Then I’d do my best to extricate myself—but I’d let him know that if your mother asks factual questions to which you have the answers, you WILL provide them.)
Other support for your mom might include letting her know about SI. If her “drinking too much” is a real concern, a complete physical and frank talk with the doctor (in case medical support is required during withdrawal; alcohol detox can be dangerous and whether she needs inpatient support should be professionally assessed—and because alcoholics downplay their intake, you might want to either join her or ask her permission to share your observations with the doc—with the goal of getting her the most and best help possible) would be wise.
My point is this: your mom’s emotional/mental well-being isn’t being well-served by secrets. They may be the actual SOURCE of much of her struggle. Truth DOES set us free:
In giving her the truth, YOU will not being doing ANYTHING to your mother other than offering her the potential for truth in her life. Yes, truth MAY involve life without your father, if he’s not willing to grow with her. (And it’s possible neither will choose to grow, too). Your father has chosen this for ALL of you. That he’s also chosen to involve you in the day-to-day machinations of his affair is a testament to his (current) selfishness & arrogance. Bringing it to light may or may not provide impetus for change. The outcome isn’t ON you. But doing the right thing is.
[This message edited by solus sto at 10:33 AM, April 17th (Wednesday)]