He is too disabled to work because of a mental disorder, correct? That may or may not mean he is unable to manage money. Has he managed money in the past? Does he have a budget? Does he know how much his rent is and would be sure to pay it every month? So far, what we know about him is that he has not demonstrated the ability to find answers out for himself since you are attempting to do it for him. That indicates that he has some limits in managing his own business.
There are payee agencies that are bonded and are in the business to manage money. They are an excellent alternative to some family members who see their SSI relative as a cash cow. You are the one who said his mother is misusing his money. She is not charging him a fee and is not required by SSA to keep receipts for everything. A payee agency is and their records are closely monitored. And, the funds in the bond are available to replace SSI funds if they misuse. For misuse to be found by your friend's mother, he would have to establish that she is spending SSI funds on things other than his rent, food, share of utilities, etc. Does she give him a place to sleep? Is there food in the cupboard when he wants it? I do believe that a mother can misuse a child's SSI and some see the SSI as family income when it is not, but there needs to be evidence for SSA to make a decision AND then it is difficult to recover the money from a poor family member.
If you think it is lame, you volunteer to manage the money of 5 or 10 friends (let alone 200-2000) and see how much time it takes you and how much grief you get.
For a story about a woman who did manage money of SSI recipients for free, read the following link. It was this story that led to payee agencies as a business.
Dorothea Puente - Wikipedia, the free encyclopedia