Answer:
The person who should sign client's informed consent for an invasive diagnostic procedure should be either relative( can be husband/wife/child) or a friend who has an durable power of attorney for healthcare of the client.
Step-by-step explanation:
Dementia can be said as a group of condition where a person is suffering by impairment of at least two brain functions ,like memory loss and un able to make decisions on there own behalf. If a person is suffering from this condition , then someone else who has the authority to make decisions for that persons well being will sign the consent for the invasive diagnostic procedure.