Forgive me if this is an ignorant question but is it perfectly okay to install a tile backsplash (in the kitchen) over plain old drywall?

The reason I ask is - I bought the materials myself for the sheetrock guy to use. I bought all the needed drywall for the kitchen walls plus hardibacker boards for the backsplash. Well, when the sheetrock guy was quitting for the day, I came downstairs and saw that he had only used 1 sheet of hardibacker on the kitchen sink wall. He claimed he didn't realize I was going to have a tile backsplash on the opposite wall too. This is the wall where the range will be. Everything is all mudded in. Will it matter if I go ahead and install the tile backsplash on just the regular drywall? I don't see it ever getting wet there.