RP, I have a story to tell, and I need some advice from other men with the same mindset.

I have two friends, let's call them Marco and Polo. Both are natural alphas, always have been, multiple women, attitude, the whole nine yards. They are also great friends of mine, since childhood. I would literally risk my life to help either, as I'm certain that both would do the same. I learned a lot from them, while comparing with trp and a lot of things matched.

So, Polo had a girlfriend on which he would see other girls behind her back. The situation is way complicated to explain here, but to keep it short both were unhappy and unwilling to end the relationship, hence the parallel relationships from Polo. I don't know if his girlfriend (which I'll call Nina from now on) had the same situation but whatever. I advice Polo on ending the relationship, and he does. That was about 7 months ago.

Marco called me today to talk and eat something. He then starts to tell me a story about he and this girl, and that it's a very strange situation. Probing a little I discover that the girl in question is Nina, Polo's ex. This is the situation: right before Polo ending with Nina, Nina called Marco to talk about the situation. She asks for discretion, which he obliges. She talks that Polo is being very distant and all those symptoms that come before the ending. Polo ends with her. Marco keeps in touch with Nina to help her cope with the ending of her relationship. It's worth noting that Nina has depression issues, their parents aren't the best parents in the world, and she has some body confidence issues (justified, not gonna enter in details here). It's also noting that a regular complaint from Polo was the lack of enthusiasm from Nina and the fact that she barely talked with anyone in the group of friends. So, back to the story. Marco begins to go out with Nina and her friends. He had an eye on a friend of her, and he made it very clear that he wanted to hook up with said friend. However it never happens. Marco sees that Nina is behaving differently from when she was with Polo (more outgoing, friendly, talkative) and they begin to grow closer to each other, and on a drunk mistake, while trying to protect Nina from another guy, Marco kisses her. As he said, they clicked. They keep in touch and end sleeping together a few weeks later, which Marco describes as a great feeling from connection with Nina and the worst feeling from betraying Polo by sleeping with his ex. I didn't ask further but I strongly suspect that they had sex more times. Marco talks with Nina and says that what is happening isn't right and cut out the sex, but they keep in touch. This was 3 weeks ago. At somewhat the same time, Polo goes radio silent with all the group. A week later, he claims that he had familiar problems and decided to lay low for a while while he didn't sort it out. Now, I don't believe in coincidences. Something inside me says that someway, Polo discovered about the situation and is swallowing all the information. Don't know if it was Nina or her friend which Marco was interested that informed him, or if nobody informed him at all, but the gut feeling is very strong. Marco called the whole relationship with Nina out, blocking her on social media and Whatsapp (what I think was a sound decision). However how we have this possible situation that Polo knows and is trying to deal with the information. I also think that Marco was a little too much out of hands, I mean, we are almost brothers. I also kinda think Marco was an asshole but this sort of thing happens.

I know this is not my problem, Marco came to me just to put it out of his system but now I'm worried about them both. What would you do on this situation?