Is it possible to know which apps are installed on other's iPhone? So, it happened that one of my known associates Mr.X installed a certain app on his iPhone. In a few days, one of his family friends somehow came to know that Mr.X installed that app. How could someone possibly know which apps are installed on other's iPhone?
If it were Apple ID information, it would contain only purchases and their dates, not whether the app is currently installed or not. That doesn't seem to be updated in Apple ID information realtime. 


Easy way to see a list of all the apps I have installed on my iPhone


So direct interaction of the two parties on the app, or in real life can be a possible reason IMO.