1. In order to control your friend's GPS, you need at least a WeHunt Premium subscription.
2. Once you have it, you need to ask your friend to share the device's admin rights with you.
See this article for how to do that.
3. Once your friend has shared the device's admin rights with you, you need to choose a dog for that specific device before you can start using it.
Please note that your friend must be the owner of the device in order to share the device's admin rights with you.