I can not say outright what your problem might be but I can share my experience and you might find a solution in that.
I publish my Apps so that they are only available in South Africa - this is because they are designed for the South African market - so somebody in a neighboring state or in another country will not find my app. How have you done this ?
If the customer (as in your case maybe) has an Apple account registered in another country (in my case, he was an iOS Beta Tester - registered in the USA) and he searches from your country then also he will not find it.
Then, another experience, if you target as follows:
#Target: iPhone, iPad
#MinVersion: 9
then some of the older iPhones (running on iOS 7/8 and lower) will not see the app as well. Maybe you did not target iPad and your customer is trying from that ?
Try searching your app on your PC and copying the URL of the App in the store and send it to your customer and see if he can find it like that or give us the name of your app here (and possibly attach the icon to help identify it) so I can search for it to see what I get. I have 3 devices (not all mine) at my disposal with iOS 8 and 11+ loaded - that should give a clearer indication if it is available.
Good Luck.