<human>: Can you give me the gist of the text in a nutshell?

My friend (we'll call him Mike) and I have known each other for over 10 years, and we're very close. He is a merchant marine, so he's out to sea on and off throughout the year for three to four weeks at a time. He's been dating his gf, Rachel, for about two years now.  
  
I just started a new job and everything has been going pretty well, until yesterday. One of the guys I work with (Carl) and I began talking about a mutual friend--Rachel. Before he found out that I was good friends with Mike, he mentioned that he and Rachel had been hooking up. He also said that he knew she had a boyfriend, and that he didn't think he was the only one she'd been hooking up with.  
  
Once I told him that Mike and I are close, he basically cornered me in the break room and said something to the affect of, "Listen man, I don't like drama or awkwardness, especially at work. But if you say something to Mike, things are going to be real awkward between us." This almost seemed as if it were some kind of social threat.  
  
I'm brand new at this job and I really don't want to be making enemies, but I'm in a tough spot here, because I feel like I definitely have to tell my friend that his gf is a cheater. Not only is Mike out to sea a lot of the time, but I also happen to know that he's paying a great deal of money on living expenses for Rachel and she's just playing him. I feel like I need to tell my friend, but I'm not sure how to handle this situation. Advice?
<bot>: Guy at my new job is hooking up with a close friend's girlfriend. Guy says not to saying anything or things won't be good between us.