Data usage while using hotspot

I have joined my friend’s hotspot . My friend’s mobile data usage is too much even without me or she using the internet . Can anybody explain what is the possible cause . I have tried all possible steps to reduce the usage like disabling background app refresh , disabling wifi assist , disabling autoupdates etc . In my iphone . Can anybody explain where is the data being used so much (like 300 mb used in 10 mins both the phones in sleep mode). Please suggest how to configure the data consumption on usage basis.