My close friend is talking to another friends girlfriend

So we have a group chat with four of us- we are all mates. It’s mostly 3 of us that are active and always messaging one another and sending stuff back n forth etc.

One of them has told us apparently that he got into a fight with his girlfriend and it got physical and he made her bleed so she is most likely filing a restraining order against him and went to the police regarding this issue. We asked him if everything is going alright with him and made sure he was stable and okay because we look out for one another. Obviously it was an issue but he then messaged me privately saying he wanted meet in person with just me and not to tell the others in the group chat (or anyone) about it.

We meet up and long story short he told me he was lying about the issues he had with his girlfriend and there was no police or arguments involved… I’m trying to process all of this and asked him what was up because he still seemed angry and frustrated.

He tells me that the other “friend” or ours who was in the groupchat and was making sure he was okay was taking this as an opportunity to talk to his girlfriend(who didn’t have a fight or anything) so this was just a test. He was messaging this guy from his girlfriends social media account which he made him to believe that they were broken up so he thought he was talking to his girlfriend when in reality he was talking i this guy who was next to me and I was in disbelief that all of this was happening…
Long story short this guy wants me to help him do something about it but as much as I want to it’s really not my relationship but they are both of my friends and I really am confused why the other person would do that…

Ps: when I asked him why he did it after him trying to act innocent to me, he said “u realise he tried to do the same to me with my girlfriend a while ago” and added “ obviously doesn’t excuse me”
And the sad thing is I believe him too so I’m really thinking if I should cut them both off or try fix this but these guys are my friends and I can’t really pick sides. Kinda stuck here 🙁