Location sharing wrong phone comes up!

The simplest and most likely reason is they are sharing the same Apple ID on both devices. They should NOT be sharing the same Apple ID.

Until they stop sharing the same Apple ID, this is what they can expect.

If they'd like to undo the mess sharing an Apple ID causes (something Apple has advised against doing for more than a decade now), they can follow the directions in this link (written by a senior member of the Forum) --> How to “unshare” an Apple ID - Apple Community

There are two scenarios: People and Devices under Find My

In the Find My app, there are distinct functionalities for tracking People and Devices. Under the People tab, the sharing of location is limited to a single device. If multiple individuals are using the same Apple ID, only one person's device location can be viewed at a time. This means that if you've shared your location with someone using the Find My app and they are also using the same Apple ID, you'll only be able to see the location of one of their devices under the People section.

Conversely, the Devices tab offers broader visibility. Irrespective of whether multiple devices are linked to the same Apple ID, all these devices' locations can be shared and viewed. This tab allows for comprehensive tracking and visibility of all devices associated with the Apple ID, enabling users to locate and monitor the whereabouts of various devices simultaneously.

Essentially, the distinction lies in the limitation of sharing a single person's device location under the People tab versus the broader accessibility to view all device locations under the Devices tab, regardless of the number of devices linked to the Apple ID.

Summary

1. Multiple Devices with the same Apple ID...
   1. The People Tab will show only one device
   2. The Device Tab will show all the devices
2. Multiple Devices with multiple Apple IDs...
   1. The People tab will show all the devices except your own which is shown under the Me Tab
   2. The Device Tab will show all the devices (If they are part of your family)

The pic below is self-explanatory