What you mention is the classic issue with Whatsapp on Android, it automatically adds the media to the phone. You can try (I haven't tried it) creating a blank text file in Whatsapp folder, call it .nomedia and see if that gets rid of showing the photos on the Photo App.
**Edit** - just tried this and it didn't work, so it's overriding this fix that works with most apps.
Thank you guys, but unfortunately I didn't solve the problem. My friend's stupid whatsapp media is still pouring on my Google photos library.
Maybe I should find new friends or fetch an iPhone to solve the issue.