If someone contacts social security and tells social security that a beneficiary isn't capable of appropriately managing their money and, as a result, their necessary needs aren't being met, then social security has no option but to begin an investigation. Sometimes the issue comes up when the person is interviewed by a claims rep - I can think of many times I suspected a person wasn't capable and started gathering evidence. And sometimes the issue comes up when a person files for disability and the medical evidence itself indicates that a person is incapable.
They will contact the beneficiary's doctor for a statement as to capability. They will take a statement from the person who contacted them as to examples of how the beneficiary is spending their benefits, they will contact the beneficiary and cover how the beneficiary handles their money; they might ask for proof that bills are being paid. If they make a determination that a payee is needed, they do NOT need the beneficiary's permission to appoint a payee. They have what is called a payee priority list and they hope to take an application to be made payee from a person high on that list. Examples would be a spouse living with the incapable spouse, a parent of a child or adult child, someone living with the payee but it doesn't have to be, someone who is in a position to be aware of the beneficiary's needs and who would be responsible in the use of the beneficiary's funds.
NOTE: As a last resort SS will appoint a professional payee or an organization - they are last on the list of preferred payees. A beneficiary doesn't have to even like the person appointed as payee as long as the payee is doing what they are supposed to be doing. That isn't normally the case but it can happen.
The beneficiary has the right to disagree to who social security appoints as payee. The beneficiary can appeal the fact that they aren't mentally competent if that is the case but social security will look at the evidence in file and make their decision based upon that evidence.
But to answer your question, the person entitled to an SSI or social security benefit does NOT have to consent if they are found to be incapable.
I was a SS claims rep for 32 yrs.