Urgent need to iCloud lock friends device

Copied from: If your iPhone or iPad was stolen - Apple Support

There is more detail available in the link.

Mark as Lost on iCloud.com/find

If your iPhone or iPad was stolen, put your device in Lost Mode as quickly as possible. This locks your device with the device passcode to prevent the thief from accessing your device and making changes to your Apple Account.

If you use Stolen Device Protection for iPhone, Face ID or Touch ID is required to turn off Lost Mode — so even if a person has your iPhone and knows your passcode, they'll still be locked out of your device.

1. Go to iCloud.com/find
2. 
   

• • You don't need a verification code to sign in, so you can sign in and mark your device as lost even if your trusted device was stolen.

1. Under All Devices, select your iPhone or iPad.
2. Choose Mark as Lost, then follow the onscreen instructions.

Here are several support articles which will help in this situation.

Read this support article to find out how to use the Find My app to locate a missing device or item. The person may not need to sign into your entire device if they are in a family sharing group with you. Click here --> Use Find My to locate your lost Apple device or AirTag - Apple Support

- A person can use the Find My app on a device owned by someone in their Family Sharing group.

Alternatively, anybody can sign into https://www.icloud.com/find to locate their device using a browser.

"You don't need a two-factor code if you use any internet browser and log into www.icloud.com/find

Note that the address must include the "/find" as shown above."

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Click on this link to find out how to 'Locate a device in Find My iPhone on iCloud.com' --> Locate a device in Find Devices on iCloud.com - Apple Support

You can remotely set Lost Mode on the device and also send instructions to erase it.

Use Lost Mode in Find Devices on iCloud.com - Apple Support

and:

Use the Find My app to locate a missing device or item - Use Find My to locate your lost Apple device or AirTag - Apple Support

What happens if you activate Mark As Lost

Mark as Lost remotely locks your missing iPhone, iPad, iPod touch, or Mac with a passcode, keeping your information secure even if your device goes missing. You can also display a custom message with your phone number on the Lock screen, and keep track of your device's location.

Once Mark as Lost is activated, Apple Pay is disabled, if you have it set up on the device. A confirmation email is sent to the primary email address associated with your Apple ID. When you recover your device, you will need to unlock it with your passcode.

Mark a device as lost in Find My on iPhone --> Mark a device as lost in Find My on iPhone - Apple Support

- How to turn on and off.

If your iPhone, iPad, or iPod touch is lost or stolen - How to find your lost iPhone or iPad - Apple Support