Learn English – proverb/idiom for “To be a lion among sheep rather than a sheep among lions”

"Silver-tongued devil" accurately describes many politicians, whose "fluent and persuasive speech" has gotten them where they are (the Free Dictionary), but to fit your scenario, I'd use it with one (all?) of your suggestions to fully capture the "backstabbing" notion : "a two-faced, backstabbing, snake-in-the-grass, silver-tongued devil."

For a single word, there’s an "opportunist,” who is someone that “seizes every opportunity to improve things for him/[her]self”), including “act[ing] as if they are close friends.” (Vocabulary [dot] Com).

To the extent that not all “opportunists” are wise enough to successfully execute their schemes, you could add a modifier such as “conniving [opportunist]” or “calculating [opportunist]”, to help your friend see what you see in his/her manager.

A similar idiom is: The blind leading the blind

It's used in a situation where someone who is incapable of dealing with a particular circumstance gives advice to another person in the same situation.