Not a great idea. Keep in mind that while cbu is not affected by being wet, it DOES get wet, and some of the can wick moisture up nearly a foot from your damp pan. Your mudbed above the liner will be damp...that's natural. Industry standards direct when using a surface applied waterproofing, it be a system, not a hybrid, and that the waterproofing extend up at least the height of the showerhead so that no water can get behind it and get trapped.
FWIW, Kerdi works just fine when applied over a single preslope layer of mud, but you need to use their drain. When you do that, the entire surface the tile is applied to is waterproof. The pan sees the most water...why wouldn't you want it waterproof? Otherwise, you'll have in excess of an inch of damp mud beneath the tile. Yes, when properly installed over a liner, it will weep any moisture to the drain, but with a bonded membrane drain, you just have thinset, and no porous deckmud to absorb moisture. Moisture doesn't penetrate all that far, and generally, evaporates in between uses. ANy small amount of moisture that does get beneath the tile on a traditional pan, gets drawn into the porous deckmud by capillary action and gravity.
Ask him to show you a TCNA procedure that says to do it that way...that's the industry bible if you want a tiled shower to work right.