Guy bestfriend is having a relationship with a married man with 2 kids. what will i advice him?

my guy bestfriend recently left and work for a cruise liner. he will be away for quite sometime and before he left, he made sure that the guy left his family and stayed with my bestfriend's parents, because he has nowhere to go and for my bestfriend to be also sure that while he's away, the married guy won't reconcile with his wife. The wife discovered this affair and the husband left their home. Now, what hurt me most was that, I know how it is to have a broken family and my bestfriend doesn't any have regards to the kids. One time, he didn't even let the married guy visit his kid that's confined in the hospital. My bestfriend hid all the resources so the married guy can't go. He also sent very hurtful messages to the wife and he even want to call the kids not to expect their father to go home anytime soon because he won't. Now, the married guy really want to get out in the relationship but my bestfriend threatened him and blackmailed him (btw, the married guy is not ';out';). Now that my bestfriend is away, he asked me to watch over the married guy who does nothing in their home with my bestfriend's parents. And I can see the pain or guilt in his eyes, could be he misses his family and he's so far away from his home. My bestfriend's parent's house is so very far from the married guy's house and it will take him many hours of travel and i knew that he doesn't have any money. I want to advice my bestfriend. WHAT WILL I SAY TO HIM?Guy bestfriend is having a relationship with a married man with 2 kids. what will i advice him?
Ask him how he'd like it if the other guy treated him the same way.... and to just let the dang guy go visit his kids....