How do you think the correspondence between Sybell and Tywin went? Did she ask for the rewards she wanted, or make an offer and wait for his reaction? As messages could have been intercepted would a code be used?

In terms of how the conversation went, I think Sybell made the first move and said: The Young Wolf has taken my castle and been wounded, he’s under my daughter’s care. I can’t kill him because his army will take reprisals on his behalf. What do you advise? 

Because Tywin was atm trying to get Walder to sign on the dotted line, I think he said: compromise the boy so that the Frey alliance breaks, and you will be rewarded.

Sybell responds: I can do that. I want a pardon for my family, a lordship for my brother, and marriages for my daughters. 

Tywin: Done. Make sure the assignation does not bear fruit – the Stark boy must not have an heir. 

It’s a short conversation, only two exchanges needed. And yes, they probably did use some form of code.