sell someone out

(verb) to betray someone

Example Sentences:

A: I asked Winnie to go on a date with me last night, and she said no.
B: Yeah, I know. I’m going on a date with her!
A: What?! You know I like her, how could you sell me out?
B: Whatever! She’s hot!

Sarah’s best friend sold Sarah out by stealing her boyfriend.

The employee felt that he was selling his company out by taking a job with one of their rivals.