Westwood isn't dumb; he's been a senior assistant district attorney for several years and has seen all kinds of cases. It's just that now it's involving him instead of handling others' cases.
Jimmy appeared at the scene during the interrogation. Although he wasn't the main interrogator, his characteristics were too obvious. After leaving the FBI, he just asked around a little and found out who Jimmy was, and naturally knew who to find to smooth things over.
In addition to his own matters, he couldn't just ignore his brother-in-law's issues, otherwise, there'd be unrest at home. So, decisions had to be made, and personnel had to be arranged in a short time, likely causing him to lose quite a bit of hair.
It's unclear what exact deal Kevin made with him, but after meeting with a lawyer the next day, no matter how Mark and the others interrogated him, his brother-in-law refused to mention anyone else, insisting he grew and used it for himself with no distribution.
