

OP you have to talk to your old manager because of the firm’s procedures or are you doing it to try to smooth things over?
If it is because of the firm’s rules, then keep the conversation focused on the facts of the new position.
If you are doing it to smooth things over, don’t bother. It sounds to me like the relationship was too toxic. I suspect that your old boss’ behavior is well known in the firm so your hiring manager may not put any stock in your old boss’ opinions.
Ok so keep the conversation to the facts of new position.