If a diesel nozzle (and filler hole) are larger, then a gas nozzle will fit. And does according to those who have done just this.
As long as it’s caught right away (as was in this case) I’d think the solution is to empty the tank with out driving anywhere – the diesel in the lines should be pretty safe. Once the tank is refilled with diesel, the tiny bit of residual gas if any shouldn’t hurt anything.
If you start driving, then yeah, It’ll get expensive.
Go go gadget shoes!!