How do we solve the problem about bad behavior (unfair evaluation) in lab? My undergraduate student told me that he had some problems when he attended the lab session in our department. This lab session is mandatory and one of the components of the course evaluation. 

The source of the problem is the lab assistants. They asked him many irrelevant questions about their student union, like the name of the chair, secretary, etc. Unfortunately, he is not a member of this union, so he could not answer. Therefore, they failed him and he has to retake the test.

I want to help my student. 

Previously, I considered reporting this case to the chair. However, I remembered that my chair once said that he did not like students who do not join the student union. Hence, I did not report to him.

Do you know the best way to solve this problem with minimum effort? 

For further information, the university is located in a developing country in South East Asia.

Note:
The union is a student organization. Historically, the union were the response for the repressive regime. To become a member of the union, the student should attend some orientation activity. Most of the activity is hazing.
From what you have written, it appears that the fundamental problem is that you are dealing with a nakedly corrupt organization. You describe your student as having been failed for blatantly political reasons that have nothing to do with their education. 

Such an action is obviously unethical, and should not be tolerated. The key questions that I see, then, are these:


Is there is anybody who you can work with in order to effectively oppose this corruption?
Does your student want to be a test case against corruption?


You describe your chair as being in favor of the current regime. You may wish to discuss in any case, as even if your chair doesn't like students who don't join the union, they may still be opposed to naked abuse of power by union members. If the chair will not act, are there deans or such who might? Are there other faculty members or student organizations who feel the same way as you? What about outside groups that might bring pressure, such as local or international media?

This does sound like an important fight to fight, but also remember that it is not only your fight. If their power is strong and you directly oppose them, the student union may choose to make life very difficult for your student, so before taking any direct action in opposition, it would be important to discuss potential consequences with your student. Since they did not simply take the expedient path and join the union, they may well be willing to fight; equally, however, they may have circumstances that make it problematic for them to do so, and it is not your right to make that choice on their behalf.