Acura Repair: Misdiagnosis., volunteer work, reoccuring


Question
Firstly, I wish to commend you for your volunteer work on this site. It is much appreciated.  My question isnt a technical question nor is it about Acuras.  I just need an opinion from an experienced mechanic on my dilemma.    

To make a long story short, I brought my 1998 boxster in because of a " Flashing Check Engine Light" and stalling engine.  The Mechanic was speculating that it was a bad coil, but wasnt sure.  He replaced the coil at a cost of $700.    

THIS REPAIR DID NOT SOLVE THE PROBLEM! Now he says the car is unrepairable because there is actually serious damage to the engine and that it needs a replacement motor.

The problem is, he replaced the coils and now I have to pay $700 for a repair that did NOTHING to fix my car!  

Am I completely responsible to pay for the repair on the coil considering it didnt solve the problem??

Any other advice for preventing this from reoccuring in the future?  

Answer
The first question is for an attorney.
The second question : You can ask the mechanic if it doesnt fix the problem, are they willing to pay for the repair?
I would fix it if I was pretty sure of the problem. And if I couldnt, there would be no charge.
There are tests that should have been tested by the mechanic before jumping to guesses. Tests like ohm readings and such.