What is the sin for which I will be held accountable, and what should I do regarding a situation in which I discovered that the nurse to whom I paid treatment fees took the money for himself and it was not deposited into the institution's account? And can I warn him and conceal his act, or must I report him, with the possibility of him losing his livelihood?

If a nurse takes money for their own private account and does not deposit it in its proper place, this is a reprehensible act (munkar). They must be advised and reminded of Allah and the Last Day. If they do not desist, they should be threatened with reporting their matter to the authorities. If they still do not stop after the threat, there is no harm in reporting their matter to them, even if it leads to the cessation of their livelihood, for they are the cause of it.