"Suspect is on standby." Kane noted to Zyan where he sat in the car with Cole, they were trailing Diego. A month was all it took for Edward to finally deliver the information about the whereabouts of Cole's mother to Detective Zyan.
Diego wasn't stupid, it seemed as if he knew that eventually Cole was going to be hot on his tail after kidnapping his mother, so he hasn't been able to go out more if not to the company. That's it!
At first, Detective Kane wasn't planning on helping them; but after the issue with Christopher Russo who went ahead to wait for him outside his house, to send a warning to him for what he did to him back at the station.
He has sent his men after him but Kane wasn't stupid, eventually everyone knew that Christopher was out for revenge so they were careful in their every day dealing so it's just basically survival of the fittest right now.