How to help my friend find her iPhone?

Your friend can use any internet browser (e.g., Safari) and log into www.icloud.com/find

Also see: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

The “trick” is if using your iPhone, to NOT click the blue “Continue Button” (which will log YOU in) …

… but rather click the link underneath that says “Use a different AppleID”

She can then enter HER AppleID and password.