How do you find the location for someone else’s IPhone

How do I find the location for someone else’s IPhone?

By their agreeing to share their location with you in your Find My app.

Find people and share your location with Find My - Apple Support

Share your location in Find My on iPhone

You can also use your phone to help a friend find their own device. Go to Find My, touch on the Me tab, scroll down to Help a Friend. This will then allow them to sign into iCloud with their credentials to locate their device.