Remus knew about madness and at that moment he should have realized what day it was, but still he didn't. The fact that he was looking at Sirius and that he had seen Peter's name on the map were just too confusing. For a moment he just looked at his former lover, wanting him to be innocent, wanting to turn back time. With Peter not dead he must be innocent. He looked around to room not seeing Peter anywhere but spotting the rat, Pettigrew was there.
He helped Sirius up while trying to calm the three young people, he had to get to the bottom of this.