Let us say that your friend comes upon a mystery novel with that very setting, or perhaps not a mystery novel, perhaps just an historical novel of the butchers of late-19th century Gdansk. Perhaps it's set in Gdynia instead.
Let us say that you investigate this novel and find that it is bad. Possibly it is screamingly bad. Possibly it is just not very good, the sort of mediocre thing one could easily read if one was snowed into an airport and one's electronics were all dead and the bookstore was full of James Patterson. ("It was so sad.")
If you are the friend, when do you find it useful to hear this? How broad a net do you want to cast? Of course one should signal carefully so that, "I saw this crappy thing and thought of your work, which I'm sure is terrible!" is not the take-home message. But when is it useful to know what other stuff is out there either opening the gates or tainting the wells, and when would you just as soon ignore the more obnoxious bits and go forward with whatever you're doing?
*I should not try to come up with random things nobody is doing so nobody thinks I am talking about them, because my brain went, "Ooh, somebody should write me that."