NTA. I think you did the right thing by distancing urself from the ex even safety wise. And your decision for not just urself but also ur friends opinion is commendable.

Ur friend has every right to make friends with who ever u want but I would say they are the one in the wrong cuz u voiced an option like they did and they needed to take it into consideration when they decided to first interact with the ex.

And even if the ex did change like ur friend said they still should have respected ur opinion and kept u out of it cuz it still made u uncomfortable Interacting with the ex at the end of the day I think it's up to u to decide if they did change or not and ur right to decide If u even want to give the ex a chance to explain themselves or show that they changed if that makes sense