Once you arrive in Hengsha, go to the 4th floor of the brothel (where you land) and interrupt an argument taking place between Edgar and Mei. Mei will complain to you about her friend's troubles, and you will be sent out to find her.
Once that is completed, Mei will want you to kill Diamond Chan OR get him arrested. The stipulation with killing the guy is you need to make it look like an accident.
When you get to his apartment, you can hear him on the phone facing the door. I waited until his conversation ended, and he went to the other side of the apartment. Then you can open the door, sneak up behind him and knock him out.
If you are going for Pacifist, make a save file after knocking him out.
After he is unconscious, drag him outside towards the ledge of the building. I know some people just dangle him over the edge and let go (which is fine), BUT you can also throw him some distance if you press the R1 button. Just aim high, press R1, and watch the bastard fly!
(and then fall fast to his death
Once the trophy pops AND you want the Pacifist trophy, just load your last save, plant the drugs on his coffee table and return to Mei. Easy peasey.