Hide Your Dagger Behind a Smile
Charm and ingratiate yourself to your enemy. When you have gained his trust, you move against him in secret.
Not as long ago as I'd like to admit, I worked with someone who was a friend of a friend. I had no reason not to trust this person, especially since I was new and this person was a close friend of my friend. However, they saw my technical skills as a threat to their own, and did everything they could to sabotage my career. Had they not boasted of it to another mutual friend, I would never have found out the source of my sudden and unexpected difficulties. I took the necessary corrective steps, and just in the nick of time, but the damage was done. The bitter lesson I have learned is that you cannot trust someone new to your circle just because they are a friend of a friend - even if you have enemies in common... They must earn it.