Continue using app after developer refunds purchase? I'm an app developer and wondering about refunding purchases.

I use Google's standard License Check library. If a user requests a refund and I refund the entire purchase price, does that prevent them from passing the license check? Or do they get to keep using the app after I refund the money?
Play Store purchases are refundable under a 15 minute window. Under that window, if the user doesn't like the app and wants a refund, using Play Store to refund the purchase will end in the user receiving back the purchase amount and the app being uninstalled from the device.

Is it possible, under the refund window, to copy a paid app to the external storage for example and then once refunded, install it from there. I'm no app developer but I know some apps that despite doing this, they check somehow the purchase via Play Store and in this case, since the user hasn't bought the app (it did a refund), the app won't work as if it was purchased. In this example the app has a free and paid version, so the paid version has more features but since the user didn't paid for that, it gets the free (despite having the full installed).