Red flag #1:
She was shocked to hear about it, and said "you better make sure BH NEVER finds out." She made jokes about it, but never told me not to do it.
No, she could not have stopped you. However, she joked about it? She never had a heart to heart with you about how you were hurting yourself by having the affair? Not a great friend.
Red flag #2:
While BH and I were separated I didn't see Sue that much....we would occasionally chat and hang out but not as often as before. She was supportive to BH, and I know reached out to him, and texted back and forth with me.
She was reaching out to your husband after he found out about your affair? Danger, Will Robinson.
Red flag #3
Months later, she suddenly told me that she was angry at me for what I had done and couldn't believe I did it...
Hmmm. . now she is playing sides? I am getting a really bad feeling about her stability as a friend. This is dangerous behavior as well.
I feel like if my BH dislikes her so much and doesn't trust her then why would he still continue to be nice to her and interact with her.
Alyssa, I was the victim of a double betrayal, so I am overly sensitive to these things, but my reading is that this person is not a safe friend of the marriage. She didn't support your marriage (by minimizing what you were doing,) and by lying to your husband, and then kind of turning on you after the fact. I think her and your husband's shifting feelings are concerning, and that there is possibility for danger there. Protect yourself. At a minimum, she is not a good and reliable friend, and I think that any contact between your husband and her should be eliminated. He should have no problem with that if his "dislikes" her so much.
I am sorry.