Sirius had been in the first group, completely unaware of what was to come. The boggart definitely shook him up - he didn't know anyone that felt good after facing one - but by the time he was watching others, he was relatively calm. Even when Remus went out there, he just clenched his fingers and hoped for the best. Remus was strong, and brilliant, and Sirius knew he wouldn't have any trouble. But then he was there, being unwound before everyone.
That alone was enough to make him stand up in the crowd. He only barely managed to keep himself from doing or saying anything. The look that Remus left with made his heart drop, or perhaps his stomach; he wasn't sure which internal organ felt like it was in the wrong place, but that really didn't matter. Sirius couldn't even think as he took off, heading in the general direction that Remus had to have taken when he left.
So much was going through him that Sirius was sure he'd get himself lost, something that absolutely couldn't happen. Then he saw him. Crouched down, shivering. Sirius forced himself to take a breath. "Moony?" The word was soft but then he was there, leaning over with his hands on Remus' back. "I'm right here, Remus."