If they claim to be NOS installers and take on that job then I would expect that a nitrous installer of average skill level to be able to setup the system on a rolling road without melting the engine. I also view the melting of the engine to be completely down to the nitrous installers incompetence and they were not operating the rolling road/monitoring the engine properly at the time.

This obviously relies on a few things, did they supply the nitrous kit brand new? Did they say they were able to install it? Did she make any special requests that they were not sure they could fulfill?

From the facts you have said then that's how I would expect a court to see it anyway.

Edit: This is how I remember my Engineering Law lecturer putting professional negligence at uni. If a defendant claims to possess a skill or profession of any kind, then they must excercise the average competance of a member of their profession/trade. Failure to do so is negligence on their part. This applies to all skills such as plumbers, brain surgeons and car tuning companies.

Obviously she would then need to find some professional witnesses to say that they do not melt every car they fit nitrous to