He says et tu Mrs. Cox. So he obviously knows she will turn states evidence, thereby betraying him. He says that because she's obviously already betrayed him - she's wearing handcuffs and being escorted by the police at that point. 
Lois: You know, I have a funny feeling that you didn't tell me your biggest secret.
Clark: Well, just to put your little mind at ease, Lois, you're right.
Ides of Metropolis
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