I'm not going to tell her I was with her boyfriend while they were on a break.
Why would you keep that information from her?
The guy told me to keep it a secret from her.
You should consider telling her. She would want to know about that kind of behavior, and you would be doing her a favor.
But me and the guy get along really well, we can be together.
You should not be with a person that already has a girlfriend. It is wrong to cheat on people. If someone was cheating on you, you would want to be told, so you should extend the same courtesy to others.