Dentist bust 8yo daughter’s nerve

Hi, we have been to the dentist this morning for my 8 year old daughter, at her recent routine appointment the dental nurse found she had a large cavity in her back molar but it could be saved, and so we went back this morning to see the dentist. I thought she was going to be getting a filling, however the dentist said her tooth is dead and it either needs a deep root clean and filling or removal but he did not recommend removal. He gave her a little anaesthetic, one injection and then began to clean. Fairly quickly she burst into tears and was in agony, she had to be very brave and get through it all and it was traumatic for both of us. The dentist told me her tooth wasn’t dead and he had bust her nerve which is why she was in so much pain, her tooth wasn’t dead but it is now. It was all so awful and I’m so proud of her for getting through it. Please can anybody tell me if this could have been avoided? I feel like I need to make a complaint and that this shouldn’t have happened?