What should a website developer do if he created a gambling program for a client, then repented, and the client returned demanding the program for which he paid, and the developer does not have the money to return to him and fears cooperating in sin by delivering the program?

If the programmer knows that the purpose of the program is gambling, then it is not permissible for him to do so, nor to assist the one requesting it by completing it, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." The programmer did well by refusing to deliver the program, and he must return the price to its owner.