I'm sorry you had such a terrible experience. I do think that the trainer 1.made an error in judgement there and 2. used some unsafe methods. I never agree with tieing a horse and "letting them get the flip out of them". If I have something to do like that, I make sure there is always a quick release. In this case, there wasn't one.
Are you responsible in any way? Hmmm... I suppose, but you did ask and pay for a trainer who you thought was qualified and gave you advice. You made the decision at the time, based on the information you had at the time. Most people don't question their trainers, just as most don't questions their doctors, lawyers, accountants, mechanics, renovators... We pay for and expect skills from professionals. In this case, you didn't get what you paid for. I wonder if he should be held liable for this?