Sorry if this isn't the correct answer place but I really really need advice. I'll try keep this as short as possible and I can answer any questions.
A while ago my guy friend (who was 15 years old at the time) met a dude (who claims to be 19) online. My friend lied to the man and told him he was 16. From the beginning, I have always thought that there was something off about the dude because he would comment on my friends post weird sexual comments. My friend would dismiss this as "silly freaky online slang" or whatever. From then to now, we have had a few arguments about our friendship especially surrounding that guy. Essentially he thinks I am overreacting and jealous that he is getting close to that dude. While also getting angry at me for getting closer to my other friends/his cousin. I let it go and decided it wasn't worth constantly getting angry over it if it will just make our friendship strained. However, I found out from my very good friend (let's call them A), that he and the guy are now dating. While this may seem like a betrayal of trust, I know A is a very very good person and I know they have everyone's best interest at heart. A told my friend to confess to the dude that he is only 16 now, and apparently the guy just doesn't care. So this means, the guy will be 20 and my friend will be 16. I'm now at a loss of what to do or if I even can do anything. But I worry for my friend's safety because he does have a history of being very open online and not caring at all about cybersafety or whatever. I know the guy lives like 10+ hours away and realistically he probably won't fly over here and kidnap him. But also, I'm not sure, i feel like there's a bunch of ways he could still hurt my friend and I know if I was making dumb decisions i would really want someone to stop me.
I've thought about informing his parents, but it's highly likely that they will not believe me over him. And this would just break his trust and also would ruin it for A who has insisted they want to stay out of it. I am extremely close with his cousin so I probably will tell her but if it's in regards to her cousins safety i wouldn't blame her if she said stuff about it. My main thing is that i do not want to involve A. And he will definitely realise who told me about this.
On another hand, I'm also a bit betrayed because we've had loads of arguments where he has reassured me that nothing is happening between them and has made me feel extremely guilty for even bringing the guy up. During these he also brings up my close relationship with his cousin and makes me feel bad about it.
I can't really confide in anyone else, because I think I am his closest friend. So that sucks. And i know I probably can't do anything or maybe it's not my place or whatever. But if anything happens to him, I think that guilt will follow me for the rest of my life.
Also side note, everything is on discord because of course it is. And also they both have public tiktok accounts where they post anime/vocaloid covers? Not sure if that's relevant.
Also another side note, I know it may seem like I'm an awful person for putting this all out there. But I am genuinely so stumped and anxious about this situation and all of it comes from my concern for his wellbeing. I don't want to lose them or see my friend get hurt.