2002 Honda Civic timing belt/ timing belt tensioner

I recently brought my daughter's 2002 Honda Civic with 97,000 miles in for timing belt replacement. In addition to timimg belt I was advised to replace water pump and two other drive belts, which I did. After putting about 150 miles on car it died while driving on highway. I had car towed to shop that did the timing belt work and they said that the timing belt tensioner failed causing car to jump timing and stall out. At first they told me that engine was not damaged and just the tensioner had to be replaced. They later called me with the bad news that engine was damaged. Timing belt was not broke but engine was. Shop said they would cover all labor to fix car. I am pretty upset thinking they were negligent in either not tightening or replacing tensioner. Since the tensioner held for 150 miles after replacing timng belt could it just fail, or did they just not tighten it enough slowly leading to damage to engine. Since the valves are bent and have to be replaced, the head has to come off is there any other damage I should be aware of? The very reason I brought the car in to the shop was to prevent somthing like this happinening and I am very upset about it. Thank you for your time in this matter. Wayne