You need to consider just how good of a friend is Cody? is he your "BFF" or just some dude you hang with sometimes? I'll advise for both situations, and the advice is radically different.
First though, before I get into this, let me just say fuck the "man code!" The "man code" is horseshit. Don't live your life based on some stupid code of so-called "honor" that "real men" are supposed to live up to. Live your fucking life based on what's good for YOU and the people you care about. That's all that matters, not some social-standard bullshit! Bearing that in mind, here's your free advice that I should be charging $1.99 a minute for.
If he's your good friend
Friends - real friends - are more important than bitches, hands down. If Cody is a close friend and you care about him, then there's only one thing to do. Rat this bitch out. Go to Cody, tell him everything that's been happening and suggest that he keep his slutty little skank whore under control. Maybe smack the little floosy around with his cock or something. She'd probably like that. Now, he may not believe you. Sometimes our bros can be a bit blinded by snatch. Tell him you'll be happy to set something up where he can walk right in and catch her being a straight nasty cheatin' bitch. Or, hell, tell him you'll record something for him to prove it (cuz I bet you could get the bitch alone).
If he's just some dude you know
Fuck 'im. Pussy is better than some lame ass dude who just hangs around that you don't give a shit about. Next time she starts rubbin' on you and shit, tell her you're poppin' a solid stiffy and ask if that means anything to her. Hook up with her ASAP and tell Cody "Sorry 'bout your luck, kid." You could maybe get some head or at least a little handy from her. She seems pretty, um.. "giving."
Now, before taking any advice, you need to consider that just *maybe* she's not really interested in you. Some girls are just flirtatious like that whether they're really into you or not. So, this whole DG letter could be completely pointless and maybe you just don't know how to read chicks as well as you think you do.
Good luck on your quest for pussy.
Evil and Shit,