How to find a friend's iPhone?

Use any internet browser and log into www.icloud.com/find using the friend's Apple ID & 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

The information above assumes that your friend previously enabled the Find My option on their phone. If this was not done, then the only option at this point is to look for the phone the old fashioned way.