SDCL § 60-3-1: Diligence and care required.

Any person who undertakes to do a service for another without consideration is not bound to perform the service unless it is entrusted to the person at the person's own request in which case the person shall perform fully. If the person commences performance, the person shall use slight diligence and care at least. In other cases, a gratuitous employee may relinquish the employment at any time.