Is it permissible for a friend who works in "Law Enforcement" to accept a paid membership from a housing association that grants him an apartment at a lower price in exchange for his help with the association's work, or is that considered a bribe?

Your friend, who works for a governmental authority with power and receives a salary for his work, is not entitled to take an additional wage, a gift, or a reward for performing his duty, because that is considered ghulūl (embezzlement/illegal taking). Al-Bukhari and Muslim narrated that the Messenger of Allah, peace and blessings be upon him, said to a worker he had employed, to whom something was gifted: "Why did you not stay in your father's and mother's house and see whether anything would be gifted to you or not?" He clarified that whoever embezzles anything from his work will come on the Day of Resurrection carrying it on his neck.