How can i find my friends stolen iphone?

Use any internet browser and log into www.icloud.com/find with his Apple ID and password.

Also see the following Apple articles for information about lost or stolen iPhones or iPads: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support and

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

If Find My was enabled on the phone before it was stolen, see what you can do in this link --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support