While it certain isn't proper for a tuner to let a customer leave without doing his best to address any problems, a certain level of responsibility must be weighed on the owner to ensure that if any problems occur that they take reasonable action to ensure that it is firstly driven safely and secondly returned for adjustments or repair inspection immediately. If you're going to modify your car and run at higher power levels there are always risks so it pays to understand the ins and outs and take responsibility. So when a car leaves the tunning shop pinging and is told its a bad batch of fuel then it goes bang who is responsable ?