Yes! A dog walker can use the Fi App with your dog, but you will need to grant them access. The dog walker will download the Fi App and accept the invite. We recommend adding dog walkers, sitters, and caretakers to your dog's account. See: How Do I Add or Remove a Dog Walker?
Please note that the Fi App should be running in the background for a walk to record. See: Why Is It Important to Keep My Fi App Running at All Times?