First of all, I suggest you do a hell of a lot more reading on this forum, as you will quickly get an idea of where a lot of us are coming from in this lifestyle. It isn't a "secret" at all, unless we chose that. You make it sound like some kind of cult.. It isn't,. Poly is an alternative to mainstream societies view on what it means to love another, create partnerships with one another. It can also can be looked at in terms of what it means to be in a family. As for unicorns and triads and other definitions that we use to describe, rather than box in, will be understood, once you have done some reading.
To the issue at hand-- I assume that in writing your story you want our opinions. A word of warning, I am totally not okay with your situation on so many levels, so much so that it makes me shake. Please realize that this is my opinion after many years in this lifestyle, and having experienced a lot in my life. I don't think I know everything, and I certainly don't think my opinion cannot be changed, I just say it like I see it and people can take it or leave it.
First of all, your friend is cheating, and that, to me, controlling man or not, is not okay. It deteriorates the relationship she has with her husband, with her kids, with herself, with you and possibly any future people that become close to her. If he is indeed controlling, then perhaps she is already sending the wrong message to her children that it is okay that she (or any others) be treated like that, and that it is okay for others to treat them like that.
The fact that she gives BJs as a way of gaining closeness to her husband and now your husband, frankly, disturbs the hell out of me. It harks of a woman that has been abused and thinks she can only find affection in people to whom she gives of herself sexually. That is a whole other topic, but brings me to the point that you two may be continuing her on that path.
It sounds to me like she is trying to get out of her marriage, and in doing so, has latched onto the two of you as a means to do that, instead of standing up for herself and doing the work for herself, a far easier route in the beginning, but filled with harder work in the future. Now she not only has you two to deal with, and oh, there will be issues, but an ex in the future that is not going to be happy he doesn't get his BJs anymore for free, that he was cheated on, doesn't have control over her, because she is in a relationship with the two of you. She could very well lose her daughter over this situation she is in.
Now, let's get to the daughter. This woman owes it to her to take control of her life and stand up for herself. She needs to show her daughter and all her kids (not to mention herself) that she will not stand to be treated the way she has been by their dad, and that she was wrong in her approach at dealing with it thus far, by thinking she could get out of her situation by cheating.
She needs to get a backbone and stand on her own two feet. How will she do this? Tell her husband that she will not be treated disrespectfully anymore. (If I were her, if she wants any integrity left with him, and possible safety, she should NOT tell him about you or her affair with you). She should leave her home, get an apartment, get a job, if she doesn't already have one, stay away from the two of you in a romantic sense, enforce some very strict boundaries on her husband around his being in her life so he can be in his daughter's, and then live her life separately. All this while dealing with the damage she may have had to her self esteem, her sense of self-worth as a person, and as a sexual person.
What do you need to do? Get your head out of the sand (out of your NRE) and think about this woman and her daughter from a different point of view. She is not someone to be obtained or shaped into a plaything that you want. She will do that and will say that is okay because she desperately wants to be loved and taken care of by decent people. It sounds like you are decent people, so act so. She wants rescuing. You are not rescuers. You have your own relationship and lives to deal with. All that I see you doing is to stop having sex with her, help her find a place, encourage her to take control of her life, hold her when she cries, feed her and her daughter when she has no money, be there for her daughter if she needs a listening ear (keeping a distance emotionally, as the last thing you want is to break her heart by establishing a bond that may be broken later), listen with an unbiased opinion to her stories of how much of an ass her husband is being and wait, patiently.... but WAIT!
In the meantime, educate yourself on battered women and poly, and take a long hard look at what fits in your life and who fits. Make a list of the kind of people you see yourself as and a list of the kind of person you would like to have be an EQUAL, as you put it, because it sounds like she isn't and might never be.
People involved in successful poly relationships have a certain level of integrity, respect for others and themselves, self esteem, confidence, way of balancing the concerns/needs of others with the needs and concerns of their own. None of you, from what you have written, are looking at what you are creating in terms of whether or not it is good for the others involved. You are simply using each other. Very dangerous and destructive. If you continue down that path, I predict it exploding in your faces and you will all be damaged, especially her 10 year old, and that is completely unfair and irresponsible.