This is a new error when my friend tried to open the apk in his phone
Thanking U in advance .
Try not using whatsapp in opening the apk.Download it and then, Open it directly from the file manger.
Ask him to navigate to WhatsApp/media/documents/filename and rename it
Now It should be something like this filename.apk.(something) ask him to remove everything else after APK then it will work sometimes it happens when shared by WhatsApp
My friend says he tried to download from the filemanager ; still it is not opening .
So send him from any thing except the whatsapp.Use teligram for example.i use it always when i need to test my apk on my mobile and it works successfuly.
Send Drive Link Or Cloudinary Link