It's painful, to see his friend hurting like this. Maybe he didn't understand the abuse part of it, but he knew what it was like to be hurting alone. He carefully reaches out a hand, moving to rest it gently on Sirius' arm, if he doesn't object to that. "It's not your fault. You didn't make them hurt you. They're the ones that are awful. They're the ones that are doing something wrong. It's not you." It's similar to what his parents have said to him, when he was upset and feeling guilty about his condition.