Hey Guys And Gals, Here is my 100% Achievement Guide & Full Walkthrough for Wailing Heights!
Once you take control of Debbie the Zombie for the first time, what we can do now is go to where the zombie is outside the fountain by the police station. (The one where we got the "Zombie Chat" Achievement).
Just talk to the zombie and he wont understand you and this achievement will then unlock
Go to 36:50 in my video to see where and how this unlocks.
Hope This Helps!