I am curious on how people fish wires after they mount a flat screen to a stone fire place? A friend of mine has this issue and ask if I could help.

It really depends on how the fireplace is constructed. Is it cinderblock with a stone veneer? Is it wood construction with a metal flue inside, and a stone veneer on the outside? Is there attic above it or a crawlspace below it? Is there a wood mantel (if so that may be the easiest way to hide some wires)?