Is it permissible to help a non-Muslim colleague who cheated on an exam and had his course canceled, and to plead with the professor on his behalf?

Cheating in exams is forbidden (haram). The cheater may be punished in whatever way the university deems appropriate, unless it was merely an attempt and not habitual, and signs of remorse are evident, in which case there is no objection to interceding on their behalf. The fact that the student is not Muslim does not prevent intercession for them, as the Prophet, peace and blessings be upon him, granted protection to Umm Hani' for a disbelieving man, and 'Uthman ibn 'Affan, may Allah be pleased with him, granted protection to an apostate who came back repentant. This demonstrates the tolerance of Islam and the eagerness of Muslims to do good for people, with the aim of inviting them to Islam, without this implying affection, love, and loyalty towards disbelievers.