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A: may be we can ask him for help,right?

B:what?that's impossible

A:why?

B:because he is on their side.(he betrayed us)

Question:

I want to use a native proverb or idiom to replace B's answer to "why?",here A and B are 'good guys' in a movie,and A wants to ask another man (a firstly good then become bad guy in the movie)
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Maybe:

"He's one of them."

MrP
Comments  
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thank you,MrPedantic
:-)

"They are the same bunch! "

"They're the same mob"

do they sound ok enough?Thank you in advance!
A: Hey! Why don't we ask that guy over there for help?

B: Man, you gotta be kiddin' me.

A: How's that?

B: Cuz he's one of their guys, dickhead.

A: Oh, right. I didn't know that.

B: Seems to me there's a lot that you don't know. Seems to me that you don't know jack shit.

A: Okay, okay...
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And the BrE version:

A: I say, Carruthers. Why don't we ask that fellow over there for help?

B: I don't know about that, old boy.

A: Why ever not, old man?

B: Well, for one thing, he's one of their chaps.

A: One of their chaps? I don't believe it! He's wearing an MCC tie.

B: You see a lot of people in MCC ties round here, old boy. Doesn't mean a thing these days.

A: Well, I'm jiggered.

B: As well you might be, old boy. Now, what are you having? Another Scotch?

A: Don't mind if I do, old man. Make it a double.