Simply turn off voicemail for calls to google voice using the web interface for that google voice number.
If you activated the carrier settings like in the image below, click the instructions to reverse those changes with the carrier. You could also call your carrier for help since it's a feature that has different commands on a per-carrier basis. They will be most familiar with their settings and order to undo the change if Google doesn't help undo it.
Without your carrier forwarding all calls to Google, you can still keep Google Voice and have both voicemail systems separate.
That will simply forward the call and let the iPhone and carrier handle the voicemail.
Lastly, you don't have to delete the google app for this to work, but that might make things simpler if your wife doesn't want to call out using the Google Voice number anymore.