Customer said cake was dry?!

Has she had your chocolate cake before?  Did she cut slices off of the cake then leave them uncovered over night on the counter or in the refrigerator,  they would have dried out from moisture evaporation.  The only thing you can do is tell her what you just told us.  That you ate the cutoffs and they were perfect and that no one has complained about dryness before then just let it go.  She didn't ask for money back, just needed to tell you what she thinks.  Let it go.  Some people are crabs, born crabby, grow up crabby and die even crabbier.  Once they crab about something it's then forgotten and they move on to crabbing about something else.  :-)

I think you hit it on the head: "dry" is relative to what the individual is used to.

Let's consider the most extreme case I can think of: If an individual is used to a tres leches cake, then almost anything is going to seem dry. If the individual is used to the sort of cake that is used as the basis of a tres leches, without the milk sauce that defines a tres leches cake, then almost anything is going to seem moist, and a finished tres leches is going to seem soaking wet (which, technically, it is).

(And yes, I rather like the Tres Leches cake served at Clifton's Cafeteria; I hope they keep it when they finish the renovations! Never attempted to bake one, though. I suspect that it probably doesn't have much of a shelf-life, even under refrigeration, and the only cake I ever baked that was devoured in one sitting was the Leland Awards cake at the Printing Museum!)

I think you meant "thanks ladies and gentleman" ;-)

You might want to start using simple syrups on your cakes.  Many scratch bakers do.