I have an iPhone 7 and I need to find my friend iPhone 6 Plus how do I do that

Are you trying to find your friend, or are you trying to help them locate their lost/stolen iPhone? If the later, then you need to use a browser and go to www.icloud.com#find and they would need to log in with their Apple ID and password to find their device.

If you have updated your iPhone to iOS 13, use the new "Find My" app. See: Set up Find My on your iPhone and other devices - Apple Support

and

Share locations in Find My on your iPhone and other devices - Apple Support

If not, see: Set up and use Find My Friends - Apple Support

What previous post wrote. But probraly will not work "after the fact." Tell your friend, go to their iCloud account from another computer and use "Find My iPhone" to locate iPhone 6 Plus.