I believe you are very close to having the app! Maybe you can write the developer a nice email asking for the Qt project, promising to port it to Symbian.
Edit: I see you aready obtained the source code. Things should be easier now.
You can try to explain nicely (as you did in the youtube video comment) how important it is for you to have this app and how greatful you'd be if he helped you. Try to elaborate a bit the part with Symbian, tell him how you feel that the platform has been abandoned.
If that doesn't work, you can still ask him for advice if you plan developing the app from zero. In that case, you should first check for the Waze API to see what other languages you need to know (java, whatever) and then get the tools and documentation.
Bon courage! (good luck!)