Your friend should have been a lot more surprised that it took a whole month for his bike to disappear in the first place. Tell him to spend $20 on a U-lock (yes, that is the cheap kind, but far better than nothing, obviously!) and call it a lesson learned.
Incidentally, I used to bring my bike into my very tiny studio apartment (it was about 300 square feet) so no sympathy for your friend there either.
Also, a crowd of people is not a deterrent for theft AT ALL - go to Wikipedia and look up "bystander effect" sometime.