Ask HN: Is it possible to reset an iPhone 6?

My sister got hold of an iphone 6; she has been trying to contact the original owner to no avail.

The iPhone is currently asking the iCloud login (I think she tried the system restore) but it won't show the owner's email.

Is it possible to do a factory reset on this? Otherwise its just a piece of art deco laying around. I dug up some shady sites claiming to unblock via IMEI but I don't know how it works these days - last time I reset a phone was a Nokia 3310...