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As K-Luv mentioned, your husband had set poor boundaries with this woman who he knew had mental problems. She likely felt strung along, given her laundry list of problems.
You are finished with her and have each other. Don't over-think this any more, though you now know you have an edge over your husband, and he can be taken advantage of rather easily.
Your boyfriend should have stopped the contact well before it got to the spending time alone stage if he really did not want anything but a friendship with her.
She was given the impression that he wanted to be with her since he spent so much time alone with her even though he "told" her he wanted to be friends only, his actions showed her otherwise.
The fault is with her and your husband both not just her.