(1) It’s actually from wallpaper safari, which is copyright free. (2) It’s very rude to go around pointing fingers, especially when you don’t have evidence. And (3), you could’ve been a lot nicer, even though you’re accusing someone of something.
Also, you maybe want to correct your terminology. “It’s not yours” can apply to everyone who gets their backgrounds of websites like unsplash or pixabay and don’t own the backgrounds. So that means they can’t use them? No, they are not copyrighted. Secondly, unless you were there when he/she found the background, you don’t know for sure they got it off google. They could’ve gotten it off someone’s drive, a background thread, or the original website mentioned.
Apologies to the shop owner for this long spiel.