Could this mean "You are a good wife, Katerina" or should "manzelka" be used in that case?
It could. Manželka would make it unambiguous.
Thanks for the reply. I'm still a bit confused as to how to be sure that "zena" would be understood as "woman" and not "wife".
As I said: "It could" be "wife".
I have a Czech friend named Krystyna. Would I say, "Krystynu, jste dobrá žena"
That Czech name is usually spelled "Kristýna". The correct vocative form is Kristýno.
I wrote Kateřina you are a good woman
Yes, do you need some kind of help with that?
What you wrote is certainly a correct and accepted answer.