I have a customer that wants a stone cave look in her shower. Is there a cement mortar based material that can be troweled on the walls and a water resistant paint to achieve the grey color of stone. I am planning to use a detramat over cement board to help water proof. Any suggestions would be greatly appreciated.

I think your customer is asking for trouble down the road. What she is thinking about will be very difficult to clean and keep mold off the surface. I think there are some great ceramic and porcelline tiles that she should consider to make something like a Roman shower. If she persists you can just trowel mortar on a hardiebacker board that has been prepped with a metal lath screen. It will not need paint but I don't think it will look too great in a year.