How to find a friend’s phone with my phone ?

If they remember their iCloud password, they can log into www.icloud.com and click on the Find iPhone app tile all the way in the lower righthand corner. Once in there at the center top, you'll see All Devices, and if you click on that it'll give you a list of any devices that were logged into that Apple ID. If the iPhone is still online and powered somewhere, you can click on that device and window will appear in the upper right corner with a few options. One of the options is to Erase iPhone. This will logout your account out of that device and erase all of its contents.

If the iPhone is no longer online, maybe because the battery is drained or it is turned off, in the center area where the devices are listed, that device will be greyed-out and show as Offline. In this case you can still click on it, and a different window will pop up with a few options. One of them is Remove From Account. This will simple unregister that device from your icloud account, so that you don't have to keep track of it anymore. There is also an option to Erase, but if the device never comes back online, then that won't work.

Here is a support article with more information: https://support.apple.com/guide/icloud-iphone/locate-a-device-mmfc0f2442/icloud