Is it obligatory to inform a colleague who received higher grades than he deserved due to a summation error, and what should I do if he does not inform the person in charge, and am I sinful if I do not inform either of them?

If you are certain of an error, you must advise the student or the person in charge to correct it. This falls under the concept of nasiha (sincere advice) mentioned in the hadith (definition) of Tamim Al-Dari (may Allah be pleased with him), where the Prophet (peace and blessings be upon him) said: "Religion is nasiha (sincere advice)." If you do not do so, you have left what is better. It is unlikely that you would incur sin based on the context of the question.