How do I turn on find my iPhone from my friends phone

You can't. Find My iPhone can only be turned on, on the device itself.

There is no way to remotely turn it on. Its a preventive measure.

If it was not turned on on the lost device before it was lost, it cannot be tracked now.

If it was turned on before it was lost, you can use the Find My iPhone App on a different iPhone or iPad or the iCloud.com website from a computer to find it.

Read here for details:

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

If you did not turn on Find My iPhone on your device before it was lost, there is no way to turn it on from another location. If you did have Find My iPhone turned on for your device, you can log into http://www.icloud.com with your Apple ID used on the phone to track it. See this for help, If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support