ETA: I should add this: I don't believe 'once a cheater, always a cheater'. I do believe people have the power to change themselves if they have the true desire to do so. Maybe your friend will realize this on her own.
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S., I can read that you have a lot of sympathy for your friend. That said, I also feel like you are buying into a 'poor me' story. I know people who do have special needs children who do maintain adult contact and adult friendships. They know how to be in relationships with other adults.
I think your friend is missing some of these skills and may have some other, deeper problems. Currently, she is only thinking of herself. She isn't willing to be faithful to her husband and she won't divorce him because it would be hard for *her*. Has she made any attempt to fix what is missing inside herself? Because at some point, when the distraction of the affair wears off, she's still going to be left being her own self, still stuck, because she's putting her energy into outside relationships when she should be focusing on her family.
She doesn't need more friends-- she needs to be honest with herself and her husband about their relationship and what they need to do to improve things. I'm a pretty good, straight-up friend to my friends and I tell them the truth, not what they want to hear. "I think you need to figure out things with your husband and your own self before you engage in another relationship. I know you are enjoying this now, but what do you really think this will come to?" I wouldn't even entertain the idea that the husband also might have a little something on the side as well--- that's a bs way of thinking and deflects responsibility from your friend. I personally have a very low opinion of cheaters... if they are willing to sacrifice a marriage to a man who supports them and works hard all week for a little fun, they will likely throw friends under the bus too. Who wants that sort of drama?
Oh, and listen, you already said that trust is a question mark in this friendship, which makes me think that you need to listen to your own internal compass. I wouldn't maintain a year-long friendship with someone I didn't trust. And if I don't trust you, you don't fall into the friend category. Maybe that's something for you to think about-- why are you so enmeshed and feeling you would be 'abandoning' her, even when she knows that what she is doing is outlying behavior? If I was engaging in activities which made my friends uncomfortable, but I knew this and yet, kept turning to them and making it their business, I wouldn't be surprised if they sort of stopped showing up. I think true friends don't put each other in the position of validating their poor choices.