How to delete multiple photos from Photos?

I had imported all photos from a friend's iPhone to my MacBook as a backup of sorts while he gave his phone for some repairs.

Nothing happened to the originals, so now I need to get rid of the copies from my MacBook. There are about 3400 photos in total (friend's) and I want to get rid of all of those without deleting my own photos. How do I go about doing it? I do notwant to cmd+click each of the 3400+ photos.

Basically, I am asking for a way to delete all photos from one import session (a particular date and time), without deleting others.

I have found similar questions, but no one seems to be having my exact problem.

Any help appreciated.