Thanks to everyone who had constructive things to offer here.
Opalescent, I think the reason I don't feel safe is I believed his judgment of people had improved since he defended the woman who threatened me physically and refused to take what she threatened seriously. This situation showed me that perhaps not much had changed.
Schrodinger's Cat, I never thought for a second that hubby would leave me for this other woman, for any other woman. What I feared was that I could no longer handle being married to someone who didn't seem to be in my corner, even though I love him deeply and he is the father of my three children.
One thing I didn't mention in my first few posts, in an effort to be brief, was that his "friend" had trashed my character to him more than once. The first time was when she dumped him as a friend after he told her their relationship was never going to be sexual. But despite that, he persistently sought out a connection with her again via email. She told him she didn't want to talk to him again at first. When she did eventually tell him that she was willing to communicate again, he didn't even tell me that they had resumed contact, nor had he told me that he had been trying to reconnect with her (and we have had a longstanding agreement to be transparent with each other about everything). I found out when I borrowed his computer and one of her messages was open there. He admitted that they had been emailing for close to a month at that point, and he claimed he did intend to tell me about it, just hadn't yet. I was somewhat skeptical since we had had a lot of problems in the past with him "forgetting" to tell me things he had agreed to disclose.
Anyway, I told him at that point that his friendships were his own business and if he wanted to have her as an in-person friend again that was fine, but I wasn't going to socialize with her myself. But I also said that I couldn't understand why he seemed so desperate to have contact with someone who had already essentially called me a controlling bitch. When his friend revisited what she had said about me, shortly after that, more forcefully and added "abusive" to her list of my character flaws, I wanted him to just block her and be done with it. Instead, he wrote to her. He didn't exactly defend me, but he did ask her to back up what she was saying with facts, things she had observed, or even things he might have told her that she misconstrued. She then sent him some ridiculous love poem she claimed to have written for him, which he showed me. I immediately googled the first line of the poem and found it almost verbatim on the web page of some cheesy New Age philosopher.
Once he realized that she was not going to even try to back up her allegations about me with anything concrete, he asked her to apologize to me and she refused. Eventually, when he told her that what was going on was causing stress in our marriage, she did send me an apology via email that felt so insincere that I didn't even respond. I just asked him again to cut off contact with her, and then he finally did.
I think we still might have been fine if he had only admitted that she behaved badly, and not spent the better part of this past year on "she didn't mean you, or our marriage, any harm, she was just upset." I heard the same stuff when someone he dated briefly threatened me with violence, years ago. And he has gotten furiously angry at me when I questioned his judgment and told him her behavior was spiteful and malicious, at minimum, and she clearly didn't care if I suffered or he did. And I have gotten so, so angry about all this too. So it's been tense, to say the least.
I am not good at dropping things, and I know that, no one needs to tell me that here. I never wanted this ridiculous woman and her tantrums to have any power over us, but she has. A few good things came out of it, we have both learned a lot about ourselves with the help of an amazing couples therapist. But I still wish I could have some faith in his judgment of people. I wish I could feel that he would really have my back if something like this ever happened again. We did make a written agreement that if any of our friends or partners in the future attack either of one of personally, without anything that looks like a legitimate cause, we will cut them off. But it bothers me that we even had to write that down, I guess. It seems like a committed couple shouldn't have to do that?