manger avec le loup et pleurer avec le berger

To pretend loyalty to whichever side suits you

Said of someone who plays both sides of a conflict, showing loyalty to whichever party suits the moment.

Il a soutenu les deux candidats en secret ; il mange avec le loup et pleure avec le berger.

He secretly backed both candidates; he's playing both sides.

Thought to be a calque from Algerian Arabic, itself borrowed from Berber. It pictures someone dining with the wolf one moment and mourning with the shepherd whose flock the wolf just raided the next.