That doesn't automatically make Nick a traitor, but it does make him a gossip. It would be much more important to know if Nick really heard it from other students or if he told Cathy personally. If she did, then Nick is the traitor because he is lying here.
MC has proven his integrity 2 times. Cathy said to the MC so he would know for sure that Rusty always takes the blame. The MC then says so he didn't know that. But he knows it because Rusty said it, but before that he used the DIK secret code for himself. Therefore Rusty knows, so that the MC kept to the code and thus respected also Cathy's desire not to tell it further that she goes. Thus the MC is excluded from the circle of the suspects.
This shows me that the DIKs are looking for the traitor. But not only the DIKs, but also the HOTs are looking for a female traitor. Rusty was distracted by a girl. John said yes, he heard that Rusty fucked a girl.