I know my BF’s buddy has cheated on his wife and I don’t know if I should get involved

Kind of a long story, so bear with me. On mobile so apologies for mistakes.

I (27F) have been dating my boyfriend (37M) for over 4 years now. Near the beginning of the relationship, he told me his buddy (now 36M) was going through some rough stuff — depression, alcoholism, etc — and to help buddy out, BF was going to take an STI test with him so he would be less worried about taking it alone. I was fine with that — we’d just started dating — I knew I was clean, best he knows he is too.

Anyway, he initially didn’t want to tell me which buddy it was in case I ever met him, but eventually told me the buddy’s name, admitted that he was married, had cheated, and was suicidal and depressed and basically a wreck as a result. BF made me promise to never let buddy know he told me all of this.

Fast forward a few years, buddy and wife (30sF) have a kid (now 2M), and BF is the godfather, so went to visit them for the baptism and I finally met them all. We had fun trip, got along, etc, and I never brought up a word of what I know about buddy’s cheating

Fast forward a few more years, BF and I go through a rough patch where there was some infidelity on his part. (Caught him sexting several other women). Buddy calls me — which he has never previously done — I’ve never talked to him outside of my boyfriend before. Buddy basically is asking me to take BF back, that he knows my BF and he’s never dated anyone as great for him as me, that BF was an idiot but i should give him another chance yadda yadda.

This is where it starts to get confusing.

I basically tell him that I know HE has cheated on his wife, so why should I listen to another cheater? In my head, it feels like these cheaters are just trying to cover each others’ backs (except buddy didn’t know about my BF’s cheating, which I believe since BF tends to hide that type of personal stuff). Buddy goes a little quiet and then goes “ummm, actually we took that STI test for BF, not for me and actually I’ve caught my wife sexting other guys, but I’ve never cheated — we’re actually in the same boat. And I’ve stuck by my wife so you should stick by BF”. I’m confused as all hell but I believe him at the time. I ask BF if this is true and he goes “yeah it was for me, I was worried my ex gave me something. I used buddy as an excuse…” We leave it at that for the time being, but I’m pissed he initially lied to me and used buddy as an excuse.

Fast forward a few months, my BF and I decide to try and make it work together, he deleted all his social media to avoid temptations, we start going to individual and couples therapy, etc. I start thinking over the initial story about the STI test my BF told me, the version buddy told me, and the revised version my BF told me… and none of it makes any sense.

Eventually I bring up that buddy and BFs stories about the STI test don’t make sense… and that im not stupid, something is fishy there, and I want him to tell me the truth.

APPARENTLY the truth is that after I confronted buddy that I knew he was a cheater, buddy confronted my BF, like “Why did you tell her I cheated?? You have to fix this — tell her whatever you need to — make her believe it or I might have to TAKE CARE OF HER”. …basically threatening to kill me if I wouldn’t keep my mouth shut. I’m shocked buddy essentially threatened to kill me… BF says he didn’t talk to buddy for a very long time after the threat, but eventually they started talking again and are on better terms again now. BF tells me he’s worried buddy is going to kill himself as he’s threatened to if wife ever found out, and has been struggling with alcohol and depression, and BF has been trying to help him through it all. However, BF is also worried that buddy might come after him or me if his secret gets out, as BF is the only one who knows buddy cheated on his wife. (but now buddy knows BF also told me).

I get BF is trying to be a good friend and help out his buddy — they’ve known each other since high school — I get he is worried about him, but I don’t like that BF is essentially protecting a cheater. And I don’t like that I’ve basically sat on this secret for 4 years now and not told buddy’s wife. I see them happy on Instagram and it makes me feel sick and guilty.

Recently I have now found out that buddy’s wife is pregnant with their second child. I am going through all these emotions and I don’t know if I can keep this secret any longer. I’ve always tried myself “its not my business”, “I don’t want to break up a family”, “what if buddy kills himself and leaves her alone” and “what if buddy comes after me or BF” — and I promised BF I wouldn’t ever talk about it (which i already broke once telling buddy I knew), but if I was buddy’s wife, I would want to know, but also she is pregnant with child 2 and I wouldn’t want to stress her out more. I have no idea what to do about this anymore.

TL;DR: I know my BF’s buddy has cheated on his wife, and buddy has threatened to kill himself or me or BF if wife ever finds out. I don’t know if I should tell her, or if it is none of my business.