Hi, Dinged.
I very, very seldomly post here (more often than not, I just don't have anything to add that hasn't been said), but I have been reading and keeping up with your situation.
First of all, I want to offer some HUGE sympathy for you and your daughter; what an awful thing for a kid to see, and read, about her mother, and what a horrid thing for you to experience.
Secondly, I want to not -quite- echo what's been said about having some understanding for your wife. Where my thoughts differ from what's been said here is, I think it'd be better said for you to have sympathy for a BDSM (especially submissive) mindset - but not necessarily for your wife. Think about what she's going through and have sympathy for her, yes (I'll get to that later), but not for what she's done.
I'm not saying she shouldn't have played as a sub for her bf. BDSM really can be great. I love it. But she did a lot of things, knowingly and also unintentionally carelessly, quite wrong to you and her family. First and foremost in that is the lying. She hasn't been honest or straightforward with you through any of this, and considering that you have been trying, AFTER HER AFFAIR, to be comfortable with her being polyamorous, withholding truth from you is unnecessarily cruel. The lying and the cheating are serious and grave offenses. The texts and emails behind your back, when she was supposed to be enjoying family time, also were just ugly and inappropriate. Then you also mentioned that she'd been using your money for physical therapy for helping her recover from playing with the bf, and not telling you that she needed the physical therapy because of the wild sex she'd had. That seems pretty wrong to me. If they played hard enough that she required physical therapy, it should be her or his money going into her recovery, not yours. (I'm not sure my opinion of using someone that hard, either, so I'll keep my mouth shut there.) For a simple example, I don't ask a significant other to buy condoms for me to use with another lover, I buy my own damn condoms. Make sense? But take it a step further - she used your money for physical therapy AND misled you as to why she needed it. There seems to be a pathological lot of lying going on. But the biggest thing that was done was the HUGE negligence in allowing access - unintentionally, intentionally, doesn't matter at all - to her daughter (or son! *It could have been him who used the computer next) pornographic images and text. NO KID should EVER be exposed to naked pics of Mommy or Daddy, let alone images depicting sex acts that they might not understand.
What I'm trying to get at is, it was okay for your wife to play sub to the bf, and have dirty things written on and said to her (though, I would go so far as to say it was wrong for her not to disclose the nature of the relationship). It was NOT okay for her to lie, cheat, mislead, and use you. It was wrong of her to use scheduled family time as part of her sexual playtime with the bf. And it was SERIOUSLY wrong to expose her kids to the material in question. I don't think she deserves any sympathy for what she's done. She's been horrifically and hopelessly selfish in ways I can't even conceive of being. And I'm no saint, that's for sure.
Now, where I DO think she needs some sympathy and understanding (I said I'd get to it).... Put yourself in her shoes for a minute. She has made some BIG, FAT, HORRIBLE, DAMNING mistakes. She is probably only now coming to understand fully what she's done. She has probably ruined her marriage, she has hurt her husband deeply and irrevocably, she has caused actual harm to her daughter through very stupid and avoidable negligence, she has caused irreparable damage to her relationship with her own daughter.... She's probably feeling despondant. Maybe suicidally depressed (I know I would be, if I'd fucked up that badly). For this, I think she could need some sympathy and understanding. Not understanding for what she's done, but understanding for how she's feeling. It's got to be impossibly horrific for her. Try to keep that in mind. Someone that ruinously depressed wouldn't take much to tip right over the edge. Be careful what you say to her. Be careful how you say it. Sure, her daughter's pissed, but don't push toward a suicide, here. Some day your little girl's gonna want her Mommy again.