Derek takes it easier than Tommy because he understands the MC better. I'm not arguing that Tommy is a piece of shit, but try looking at the situation from his perspective. Tommy thinks the MC is just like him—an ordinary guy who wants to fuck as many girls as possible. Would you like it if your sister slept with a guy like that?
As for the secret code, as was rightly noted here, no one has ever established where the upper limit is, and this leaves endless room for interpretation. Imagine if one of them came along and said, "Okay, guys, I killed a guy and I'm going to bury him in the backyard, but you can't talk about it, because I use a secret code."
It's hard for me to say how much Tommy cares about Josy, but judging by the situation with Oscar, he does care about her to a certain extent. He might even consider her a member of his family. And he probably thinks that a secret code is not important when it comes to someone close to him. And I agree with him. If the well-being of someone dear to me is at stake, I'll throw any secret codes to hell.
