It will cost a fortune (for the disposal of the mixed fuel) if they have driven it it will probably need a full fuel system flush and clean possibly new fuel injector rail, and perhaps a new CAT converter if its knackered..
bet they wont do it again
Best thing to do if you realize you have put the wrong fuel in is NOT TO START IT! I dont think Fiat have gone down the fuel pump prime when unlocking route so you'd be fine unlocking the car and turning it ONE click to release the steering lock and push it out of the way and call on the nice AA man :worship:
On some cars the fuel pump primes when unlocking if thats the case don't unlock the car!
I'd assume that they had put unleaded in a Derv as it would be almost impossible to put Derv in a petrol due to the filler neck restriction