Can a collection agency garnish my S.S.I ckeck?

My brother is on SSI. He receives about $700.00 a month. He was disabled in a car accident many, many years ago. He has a collection agency after him for some ambulance rides he had to take to the hospital. He has very limited use of parts of his mind. I suppose you could call it fragmentation, some times, somethings, he remembers other things not. Bottom line, he would be the village idiot. I told him they couldn't garnish his check. To take it and stop direct deposit. Am I right?