Well this sounds like a sticky situation. I am going to do the best I can to give a different perspective on this. Bare in mind that I am giving you insight from an outside observer that knows only what you have shared. A little about me real quick so you can understand where my opinion is coming from.
I am a 26 year old male that was with the love of my life for 5 years. I take a long time to trust others due to my upbringing and a few other external factors. If I get to a point to actually trust.. I am a lot more vulnerable than most people because I am not used to the feeling. My ex ended up cheating on me and decided to attack each vulnerability specifically before making her exit. I never got any answers as to what happened to make her flip 180 and warrant such an attack before she left. Needless to say, it fucked me up hard.
Now I don't know what happened to your guy friend to fuck him up or know how fucked up he really is. But I see through a fucked perspective so I am going to break down what he did through my eyes. As this sounds eerily similar to what I do (unfortunately). So here it goes.
Things start out great with a girl. We have the buzz of a new relationship and nothing could be better. I am the normal me, and people really like the real me. We can discuss relationship type things such as what we want, future, and whatnot. Things are cloud 9. Until she actually shares her elevated emotional status. BAM. Shit just got real. Literally my entire view of her changes. I will be the first to admit, it is NOT her fault. She did NOTHING wrong. I cannot handle the thought of a relationship. I instantly (seriously.. instantly) devalue her. I remove any emotional attachment and the only thing I can see are flaws. This is MY messed up psyche, NOT a reflection on her. I'll cut her out completely. I cannot handle it. It's as if I was starting a relationship with my ex, with all the knowledge of what was to come.
Of course this "protective bubble" of devaluing her will ease up a little bit. Allow me to actually process what happened. I am able to see how stupid and twisted what I did was. I can see all the things I really liked about her again. This makes me sad and I reach out. As I am the one who broke it, I feel I owe her an explanation. Whether it is the whole truth (rarely) or enough information so she can understand why. This generally has an unintended "consequence". I just opened up and allowed myself to be vulnerable. Generally women like this, when not overdone. I now have to constantly and consciously fight to keep my "protective bubble" at bay now. A constant and tiring uphill battle. Sex makes keeping this bubble at bay easy.. for a while. I have no idea why and I am kind of ashamed of it. Since I feel like I am using her. Eventually it's not enough and she is a stranger to me again.
Feeling like an ass hole and realizing I am messed up. I just leave her alone. I will ignore her as if I am doing her a favor. Protecting herself from me. At least that is how I rationalize it. Generally at this point she just gets fed up and leaves. Which in my twisted mind, validates my actions.
The time line of what he did to you and what I unfortunately do to others is pretty spot on. Granted there is no way to know if he thinks like this or if he has other reasons. But the time line struck a chord with me.
As far as proper etiquette to ending FWB, I am obviously the wrong person to offer advice. The only thing I can say is that if a woman approached me in a non-emotionally-charged way. I could actually discuss it. But at the first sign of anger.. I shut down. I am ok with sadness.. but as long as she isn't blaming me.
I hope you can glean something from this. Again this is just the situation from my perspective. No way to know if him and I have similarities. I think you should talk to him. Just be neutral and make your intentions clear from the get go. By that I mean "I am trying to figure out what happened." Be neutral as possible, despite the fact you're emotionally invested. I wish you the best.