Use iPhone as Data Source for Standing

You haven't shared any exclusive data from the Apple Watch on your iPhone. That speaks volumes. There seems to be some misunderstanding about how iPhones track standing hours.

• iPhone cannot track standing hours on its own: Unlike steps that can be detected by the iPhone's motion sensors, iPhones lack the necessary tech to reliably identify standing specifically.
• Apple Watch is the key: Standing hours data is exclusively collected by the Apple Watch's accelerometer and motion sensors.
• iPhone as the data source: In your case, the data displayed on your iPhone's Health app for standing hours likely originates from your Apple Watch. The iPhone itself is just presenting the information.
• Girlfriend's iPhone: Unfortunately, there's no way for your girlfriend to add her iPhone as a data source for standing hours. An Apple Watch is required to collect this data.

In summary:

• iPhone 15 Pro displays data from paired Apple Watch.
• iPhone itself cannot track standing hours.
• An Apple Watch is required to collect standing-hour data.

You haven't shared any exclusive data from the Apple Watch on your iPhone. That speaks volumes.