...
" . . . before they have a chance to stab you in the back."
" . . . because you never know when they'll need to use you too."
" . . . because foolish use of friends is foolish."
" . . . and your enemies even wiselier." [Yes, I know it's not a real word.]
" . . . because taking advantage of friendship is a delicate task."
Meh, so I won't win any awards for proverb-writing. Sue me.
~
Someone named Garfield coined it.