For the source material please read the Radiolab segment, as well as the two articles on Dr. Gey's use of Henrietta Lack's cells from the biomedical test chapter 3.
Answer:
Dr. Gey's use of Henrietta Lack's cells after 24 years was not unethical due to his effort to figure out what was wrong with cells and why it happened to Henrietta.
Even if he did his intentions were to figure out how to treat cancer and help many people from the lethal condition. His effort made it possible to grown immortal human cells in culture. That help in developing the polio vaccine, and many other milestones.