Is it permissible for my friend's father to seize an apartment owned by the project's broker, as compensation for the money he lost in the project after its owner disappeared, with his intention to return the apartment to the broker upon his return and claim his right from him?

It is not permissible to seize the broker's apartment. The claim of deception must be proven before the judiciary to ascertain the truth of the matter and the extent of the right, if any. A person should not be granted something merely based on their claim, for the Prophet, peace and blessings be upon him, said: "Were people to be given according to their claims, men would claim the blood and property of others. But the burden of proof is on the claimant, and the oath is on the one who denies." It is not permissible to assist in this through forbidden false witness, which is considered one of the gravest major sins, for the Prophet, peace and blessings be upon him, said: "Shall I not inform you of the greatest major sins? ... Associating partners with Allah, and disrespecting parents... Beware, and false speech, and false witness." So, let whoever claims injustice turn to the judiciary to obtain their right.