I want to chime in as somebody who has been in your boyfriend's position but not with nearly so crazy of a person. When I was first dating my boyfriend (in the time range you guys are at right now) we were very serious. I've known he was "the one" since about four months in. It's been three years and nothing has changed.
But, I definitely had a few guy friends that I didn't tell we were dating for a while. This is because those were my guy friends who it was "complicated" with. And since I had made a recent move, they weren't around to personally see things. I didn't hide my relationship from them because I was trying to keep them on the back burner to possibly date later. NOT AT ALL. I didn't tell them because I knew it would cause them pain, and it would probably ultimately end our friendships.
I've lost two of my best friends of many many years over this relationship, just like I knew I would. One was an ex-boyfriend (we will call X) who didn't have feelings for me anymore, but still had a weird need to be relevant in my life. My boyfriend demanded that I cut contact with X, because my boyfriend wasn't comfortable with X's behavior, and was shocked that I was comfortable with it (the boundaries/spine thing you are mentioning). So I did. That sucked. I lost a good friend and a previously important person in my life to make my boyfriend feel more comfortable.
The other one was a guy who has been in love with me since we were kids (we will call him Y). I've never been interested in him romantically and tried to nicely tell him that many times. But, Y and I had a drunken sexual encounter a while before I met my boyfriend. Afterwords, I told Y that I wasn't ready for a relationship right now (I didn't want to hurt his feelings). Then, I started dating my boyfriend, which I knew would hurt and upset Y, so soon after I said I didn't want to date. I knew the final stern talk "I don't love you. I have never loved you. I will never love you." would happen now, and that he would actually have to listen for once. I wanted to forestall this because I really valued Y's friendship, even though it seemed he only valued me romantically. I was right. I haven't heard from Y, my best friend for a decade, in three years. That was what I knew was coming and that is why I put off telling him.
That is what your boyfriend knew was coming too. He knew his friendship wasn't going to survive with this woman, and it didn't. Aside from her crazy romantic feelings for him, he obviously once had a FRIENDSHIP with her that was meaningful to him. He lost that. He knew he would and was trying to forestall it. It seems crazy to our partners that we deal this behaviors, but they don't see the core of actual long-term friendship underneath the crazy on top. They don't see that we've had 999 non-crazy interactions for every 1 crazy one, and expect us to drop the friendship like a hot potato because it's making them insecure in this moment.
Now, not saying that you need to be steamrolled. Obviously in my two examples I did end up choosing my partner. But my partner didn't just snap his fingers, say "I'm uncomfortable" and I kicked them to the curb. We had multiple long, respectful talks about where the boundaries were in our relationship and what was healthiest for me, him, and us as a couple, and for my friends. Not everything is black and white.