Harry, Hermione and Ron followed after Mayumi as she casually strolled back into the castle, (Lupin is off in the forest right now) and just knocked on Professor McGonagall's office.
"Good evening Professor McGonagall!" She chirped as their head of house opened the door, "This rat is an animagus named Peter Pittigrew, he is a serial killer that framed Black. Please turn him back."
The next day, Black was declared innocent and the scum rat is in jail.