I would start polling masons. One of the problems with massive stone slabs = cracking/breaking as it tries to expand at different rates. A solid slab will have temperature exposure that is uneven. That stone will try to expand in different ways in different areas as it heats. A solid stone will have nothing to do but crack or break at some point in it's life. And when it does, it is VERY exciting/explosive. Sometimes these will split and shards will come shooting out. I've seen this happen around camp fires when large stones are used that were placed too close to the fire. It can be very scary. This would be my concern.

Stones (to a certain size) have been used in lieu of brick...but that doesn't seem to be what she is looking for.

I would start dialing all the local, HIGH END masons in your area as well as chat with fireplace experts to find out if this is allowed/feasible. And if so what the COSTS will be to get it to work.

Then start looking at inserts that will offer her the aesthetics she is after without the concerns of a massive stone slab. There's a reason why brick, stone and mortar are used for fireplaces...or metal inserts. I would be prepared to explain to her that the "idea" is not an advisable option (if she insists please make sure she is offered a waiver to sign so that NO ONE is responsible for damages or issues that her decision brings about).

Good luck. Make sure you bill for your time.