My friends phone was stolen how do I find it

Use the Find My app to locate a lost or stolen device - https://support.apple.com/HT210515#locate

A person can use the Find My app on a device owned by someone in their Family Sharing group. Alternatively, anybody can sign in to iCloud.com/find to locate their device using a browser. Click on this link to find out how to 'Locate a device in Find My iPhone on iCloud.com' --> https://support.apple.com/en-euro/guide/icloud/mmfc0f2442/icloud

Have your friend click on this link and read it to find out what to do "If your iPhone, iPad, or iPod touch is lost or stolen" --> https://support.apple.com/HT201472