If you are the account holder, you don't have to have the device and the user does not need to agree to install anything on their line in order to get this set up.
It can actually be done in so called 'stealth'. The only tell though, is that they won't get their voicemail the traditional way they do now. They would instead call their own # and enter a passcode you set up (because they will be using Google voice for their voicemail once you set the Google Voice integration up). I haven't found a way around this particular 'tell' yet.
If you want more detail let me know.
Kind of like using Sprint Family Locator without the family member user that you are locating knowing and getting the notification text..it is possible.