How do you fire-block a fireplace?
I am not exactly sure what your engineer friend means by fire block. He may have meant fire brick or even a flue liner.  In either case, before you use a fireplace for the first time with an unknown history, it is extremely wise to have it professionally cleaned and inspected with a camera. There could be several conditions that can be dangerous, including loose or missing tile sections, no tile at all, dried out and missing mortar joints, improperly sealed thimbles, just to mention a few.