I agree, core is to dark and you can't stain the grey out even though the shade is sooo close......Adding white modifiers may work but that would be a lot of trials & errors..
To get this monkey off your back, I would go to the office with a GC Lustre kit and toaster oven and stain with his lighting conditions, and get the patient & doc to agree its the best that can be done. Losing time is more than worth another remake. It also lets you see if you really need to remake the frame and start all over. It also lets you take the shade with your shade guide and translucent/enamel shade tabs if your have them for your porcelain system !
Sandblast the surface to get rid of any traces of stains before using Lustre.
Privately, I would tell the doc he needs to start using the 3d shade guide on lighter anterior cases to insure the value is correct. Explain to your account how it can help to take stump shades, and also value shades if you aren't sure using only the classical shade guide.
From the picture you posted, this is a perfect example of why you must have a 3D shade guide and consider the value first.
Great work on your part !