jackatthebump - A fake friend who fakes your friendship until you're out of

A fake friend who fakes your friendship until you're out of luck and they can laugh at you.

'You failed the test? I passed it. I'll pretend I didn't know you.'

'He asked me to his party, then told everyone I was a weirdo.'

'He sat with me at lunch, then told everyone I was a loser.'