I am a mortician. I have received the deceased's brain in an unlabeled box before. The context was a suicide by gunshot to the head while sitting in his car. The cops called to let us know they were "clearing the scene" and found "additional brain matter". It ended up being about a quarter of his brain, and a large chunk of skull. I placed the brain in his chest cavity with the rest of his organs (post autopsy, the organs come in a bag that's sewn into the cavity after the embalming). I used the skull fragments rebuilding his head.
If the package had arrived after he was sewn up, it would have been a lot trickier to deal with. I would probably have placed the box in the casket right before it was closed for the final time, so it could be buried with him. If it arrived after the burial....damn. I guess I would ask their permission to cremate it free of charge, and either return the ashes to them, or scatter them by the grave? That would suck.
There is no scenario where I would ever ever ever hand something like that to the family. I don't even give them back bloodstained clothing. I'll let them know it's soiled, and if they want it back, I wash it well first. Our whole job is to try and help people who are grieving, and to protect them from unnecessary trauma.
Whoever gave this family that box needs to get out of the field, and go do something that doesn't deal with people.