What should I do if someone else's passport is detained by the Immigration Bureau at Brunei Airport?

Go to the Embassy of China, and they will take care of it for you.

Because you are a citizen of China and China, you have to go to the embassy when you are abroad. . .

That's an international practice. If you are repatriated and blacklisted, you are not allowed to enter this country.

It's as simple as that (this is the lightest) because they have no right to detain you all the time.

And if your behavior violates local laws (fraudulent use of other people's passports is a criminal law in some countries), they have the right to sue you according to legal procedures.

If a criminal offence is involved, you may go to jail, and the Embassy of China has no right to interfere.