Change ringer remotely on a friends phone who is on Find my Friends

There is no option to turn an iPhone ringer on remotely. However you can have it play a sound even if the ringer is off.

Have the friend use any internet browser and log into www.icloud.com/find and select the phone and click Play a sound.

Also see the following Apple articles for information about lost or stolen iPhones or iPads: Find your lost iPhone or iPad with Find My - Apple Support and

Find your lost Apple device or AirTag with Find My - Apple Support