To enter into a legal transaction on behalf of another person, you need the authority to do so. This is usually provided in the form of a written power of attorney (POA), which authorises you as the agent to act on behalf of the person giving the power (known as the person) within the authority contained in the POA.
In terms of law, an agent cannot perform any act which the person has no legal capacity or mental soundness to perform. Therefore, if the person who gives the POA when he or she is of sound mind, later suffers from a mental illness or severe or profound intellectual disability, the authority granted in the POA ends.
An agent cannot perform a legal act which the person cannot do himself/herself.
For many years, the Law Commission has been investigating enacting legislation to introduce an enduring POA, which continues to be valid when a person becomes incapacitated; or only comes into operation when the person becomes incapacitated, but this does not currently exist in South Africa.