Is there a saying or proverb for when a person or group of people act politely and with respect towards a certain member of a group in front of a person of respect or elder, and then acts with disrespect and completely differently towards that same member of the group when the respected elder is not around? It involves being two-faced and a kiss-ass. "When the cat's away, the mice come out and play" isn't a good fit, yet has some elements of what I'm trying to describe. Is there a saying in another language that might work?
If your audience consists of those conversant with classic American television, this type of behavior could be described as being a regular Eddie Haskell (or "acting like Eddie Haskell"). Eddie Haskell is an archetypal character from the classic early TV show "Leave It To Beaver" whose defining characteristic was exactly the type of behavior you describe.
I'd like to suggest you combine an adjective denoting "two-faced" and a noun denoting "kiss-ass."
Adjectives for two-faced (besides "two-faced" itself):
Nouns denoting kiss-ass (besides "kiss-ass" itself):
Thus you can generate a variety of epithets, like:
Barry is such a backstabbing sycophant.
I can't believe what a lying booklicker Sheila is.
Don't be such a deceptive flatterer. Tell the boss how you really feel.