[Question] about external HDD games and using it on other PS4

Hello, I need some help because I'm going to friend house and want to know a few things about using my external HDD in his PS4.

First of all, I understand that I have to put my account in that PS4 and set it on primary (no problem, close friends) and just connect the HDD.

But I'm wondering a few things

1) I have Mortal Kombat (disc game) installed in the external HDD, my friend have the same game on disc, installed in his internal drive (he doesn't have a external HDD). If I connect my external drive, what will happen? will it overwrite any of the 2 games?, will it delete it from one of the storages?

2)The same question to digital games, really don't know if there will be a difference between disc and digital.

Thanks :)