My advise to the lawyer plaintiff, use your head & get some ingenuity, if that's not oxymoronic. Makes friends w/ the perp. At the Ansonia, if the roach producing tenant is old, they are rent stabilized, and if gone the building could rent/sell her apt for much much more than what they're paying.
Had this problem years ago and solved it, but I'm not a lawyer, which may explain why. While living in a walk-up I started seeing roaches in my very clean apt. Couldn't figure out where they were coming from. One day a very elderly lady who I never seen before cried for help in the hallway, literally, "Help, help, I can't get up". I helped her upstairs to her apt, she wasn't hurt, and upon opening her door saw roaches crawling on every surface. The walls and floor were moving. I decided then and there I going to become her friend. She complained the landlord comes in to spray but to no affect. She was convinced he was spraying water so no cure could be had, she would have no choice but to move, and he could then rent her apt to a much higher paying, non-rent stabilized tenant. I agreed. I asked if I could help her w/ it, she said yes, please, and was very happy there might be hope. I bought several industrial strength bombs you can only get at a hardware store in Spanish Harlem. We made a day if it. I helped her outside to sit on the stoop, where she really enjoyed seeing people she hadn't in years- apparently she only came downstairs to get her mail and had everything else delivered. I set off three bombs, and waited a couple hours, then went in w/ the concentrated spray from same hardware store. Opened the windows, and the lady came back after the apt aired out. Problem solved. Never saw them again. I was happy. She was happy. She turned out to be a very interesting person, a reporter who knew Ben Hecht, met Marlene Dietrich, etc. Very smart, very cranky, very NY, but sadly, very old. Could do little for herself. NYC is very hard for old people.