What do you advise me to do if I see someone cheating on an exam, given the differing fatwas between the obligation to inform the supervisor (as Sheikh Ibn Uthaymeen holds, based on the hadith "He who cheats is not of us" and "Support your brother whether he is an oppressor or oppressed") and the absence of such an obligation?

There is no contradiction between the obligation to change an evil (munkar) and the pronouncement that silence about it is forbidden, along with the clarification of the stages of changing it. One begins with direct advice, and if that does not work, the responsible authority is informed to change the evil, but on the condition that it does not result in a greater harm or corruption. If one fears that, then a person may denounce it with their heart, and there is no sin upon them; according to the hadith (definition): "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." Thus, advice is obligatory to the extent of one's capacity, and its obligation is waived if the one advising fears harm to themselves.