The one who is the root to all this is the woman, isn't she? I am not entirely sure how to confront her, and I do acknowledge that it may be difficult to do so. But I do believe she needs to make a decision rather soon, one way or another. Convincing her to "stop things" isn't necessarily the best thing, but at least press that button to try to make her feel guilty about what she's doing. As in, trying to talk to her in order to make her make a decision who she wants to be with. Because... she quite plainly cannot have both unless her husband agrees, and I quite doubt that to be the case. Then again, you may be misunderstanding something, in which case she should be given the chance to defend herself. ... Not saying that defense might not be filled with lies, but in either case. 

I'd say call her, tell the house that you want to speak directly to her, and then try to make that happen somehow. Then ask her, speak to her and press her about it. Of course, then there's the question of if she'll react like a sane human being or like a child. That's what I think. And perhaps give your father a piece of warning, but yeah, it is her fault to begin with so beyond a warning shouldn't be necessary.

I'd also like to mention I have zero relationship experience and the above is all from watching love-dramas, but in any case. I felt like I should make my input. That's all.