Update;
As you can imagine, I didn't leave good feedback. I wasn't rude and I didn't insult him. If anything, I think I denounced him in a kind way.
Well, now he's begging me to change my feedback. I was never going to but instead of jumping in, I thought about my reply for a day or two.
Basically, I told him that my feedback was honest and that I wasn't going to be dishonest and give good feedback.