Should a mechanic Carry out a diagnostic scan on a car before starting work on it to repair the problems?

Hi Guys I need some independent advice as I am due to go VCAT regarding a Mobile Mechanics company called Ace. They attended my car on the 13th May as it was not performing very well and sounded and felt wrong. The mechanic did not scan the car with a diagnostics report at the start and instead went off to buy new parts as he believed it needed new spark plugs and a coil. When he returned he replaced the items and jump started the car which would not start. He charged us $700 and told us to get it towed to a garage. We took the Car to VW to have a look at it and was told that the ECU unit was not working. I am reliably informed that a car cannot work without a fully operational ECU. The mechanic drove the car up the forecourt of my house at the start but could not get it started at the end of his visit.
My problem is that I need to prove that they are negligent and were responsible for breaking the ECU as they deny any responsibility. Can you please confirm if it is normal practice for any engineer to carry out a diagnostic scan on the car to find out what the problem is. I was told by VW that this should have been the course of action although they cannot / will say that the mechanic was responsible for the damage to the ecu unit they say that it is unlikely to be anything else.
I have a VW Beetle 2002 and I am told that it would also be very rare for the unit to break. Can you give me an independent answers to the following questions please:
Is it likely that the mechanic damaged the ecu when jump starting the battery?
Should he have carried out a scan prior to starting work?
If the car was operational before he started work on the car is it possible for the ecu to stop working after he has carried out his work?
I really appreciate your support in this matter as I had to pay over $5000 to get the car repaired and it turned out that all it needed was 4 new coils.

many thanks Jenni McFaull