No contest on that. That can actually be very dangerous for the diabetic in question, especially if they are type-1. (There really ISNT any insulin! Not the same as insulin resistance type 2, where there IS insulin, it just is not working as it should be)-- Especially when presented in liquid form, along with a metabolic stimulant that accellerates delivery to the bloodstream. That has "Can maybe cause a coma" written all over it, especially if the diabetic has not taken their insulin jab yet, or is close to the time they would need to take it.
I totally "get" that sugar substitutes simply do not sweeten the same way sugar does. (Most are many times sweeter than sugar, lack the warmth of sugar, lack the mouth-feel of sugar, etc...) and that the barista might be a coffee snob that insists that these things be "Right"-- but that isn't grounds to possibly be imperiling the customer, when they ask for a sugar substitute. Instead, the barista needs to have a good grasp of how to balance the harshness of the artificial sweetener to get as close an approximation as possible.
prejudices are problems everywhere, and everyone has them. Be it "skinny bitches", "Turtleneck hipsters", or any other stereotype people have.
On the flipside of that sordid coin though, I feel it is important not to go TOO far into the PC direction, since prejudices are personality faults that help define who a person is, and what their personality is. I am against reprogramming people in any form, even when they are biggoted a-holes. They have the right to be biggoted A-holes. What they dont have the right to do is harm other people. It's a delicate line.