In Italian we have a sentence,
passare alla concorrenza
It means that someone who worked for company X starts working with company Y, who is a competitor of X (they sell the same goods for the same market). This sentence can also mean that, say, a TV host starts hosting shows on another TV channel, etc.
Can you help me find an English collocation with the same meaning, if any exists?
Let's say Bob first worked for Uber, but now he works for Lyft, their competitor.
Work for the competition seems clear to me in this context. Using Bob as the example,
I'm disappointed in Bob. I heard he's working for the competition now.
Here's just one example usage I found online:
Sleeping With The Enemy: Tech Employees Who Go To Work For The Competition
As for an idiom, how about jump ship?
jump ship
a) to leave an organization that you are working for, especially in order to join another
The best employees jumped ship at the first opportunity.
(Longman)
Using Bob as an example,
After all those recent scandals, Bob jumped ship and now he works for Lyft.
There is an idiom that fits:
Now she's playing for the other side.
That is, she is playing for a rival or competitor.
This idiom is used where I live. This happened this year, where I work and someone went to work for the competition or the enemy.
He is now working for the enemy!
Some define enemy simply as not a friend.
Enemy (n.)
Look up enemy at Dictionary. comearly 13c., "one hateful toward and intent on harming (someone)," from Old French enemi (12c., Modern French ennemi), earlier inimi (9c.) "enemy, adversary, foe; demon, the Devil," from Latin inimicus "an enemy," literally "an unfriend," noun use of adjective meaning "hostile, unfriendly" (source also of Italian nemico, Catalan enamic, Spanish enemigo, Portuguese inimigo), from in- "not" (see in- (1)) + amicus "friend" related to amare "to love" . From c. 1300 in English as "adversary of God, unbeliever, heathen, anti-Christian;" late 14c. as "the Devil;" also late 14c. as "member of an armed, hostile body in a war, feud, etc.;" of the opposing military forces as a whole, from c. 1600. From mid-14c. as an adjective. - The Online Etymology Dictionary
Even Trump has used a similar phrase about the democrats (or if you wish the competitors) as seen in this article: Trump embraces ‘enemy of the people,’ a phrase with a fraught history.
"turncoat" might be the word you are looking for if you mention betrayal.
"Rat" might be the word if you prefer something rather informal and insulting for a person who changes loyalty.
"change sides" can also be what you are looking for.
