They had to separate the tie rod end. It probably was tight so they used a pry bar and a shop hammer on the side to jar loose. Either the pry bar slipped or the hammer glanced off the knuckle arm.
One thing to consider, although I am not making excuses for the response, is that it is very likely the same thing could have happened if any of us was doing the work. We do not know what the tech was dealing with in tearing it down and sometimes things just happen.
The question becomes who is responsible for the repair? If you had done the work yourself obviously you would have to fix it. If the owner of the company was not a idiot he would have had a conversation with you about a solution.
That said, I had a customer one time that brought me a piece of junk car to work on. As I was attempting to repair everything I touched broke. So I called him and said it was going to take more money than originally quoted because of all the issues. He went crazy yelling and screaming it was all my fault. So I pushed his car out into the street and told him to call a wrecker or I would. That was that.
So a lot of times the response from the customer can cause a negative outcome. I am always willing to work with someone willing to work with me.