How did you realize? After the strange engine work? Did not he have the engine damaged?
Pump out how much you can for example by connecting the fuel pumps in the tank with cables to the battery. (I'm guessing that diesel 2016 year has electric pumps in the tank).
Pour 10l of diesel and pump out again.
Fill the tank full of diesel.
A small amount of gasoline in diesel will not damage the engine.
Some even add gasoline to diesel in the winter in my country. For example, when they still have summer fuel in the tank, tractors, combine harvesters or when the frost comes suddenly.
I think (maybe incorrectly) that in Australia, passenger cars are almost always gasoline so such errors happen rarely.
In my place it's 50 to 50 so it happens often.
I know several such cases personally, authorized dealers rinse but never dismantle tanks.
Another matter, as Xcentric wrote, is a guarantee