I only seen the beginning of this thread yesterday and not what it deteriorated to later on. In order to keep it on track and possibly help someone else now or later on, I will respond this way.
Several issues have to be looked at with several already mentioned. What is the actual cause of the damage? A failed injector will be different than the damage from a failed piston. Also, one injector out will not likely take a whole engine out.
No matter how one looks at it, the dipstick issue should have been addressed. If the dealer failed to put in engine oil, that is equally shared problem. If a garage was not going to check it, then the drivers and the owner should have been told with a confirmation of that information on the receipt releasing them from any liability. If that was done and the drivers left anyway, there is really no recourse.
At the end of the day, the dipstick should have been addressed. Especially since it is such a inexpensive but important thing to have.
No real winners in these kind of experiences. Hopefully it can be put together and get the experience behind you.