What is the penalty for the dummy failing to follow suit?
The actual answer is in Law 64D 


  there is no trick penalty for the established revoke ...3. if the revoke was made in failing to play any card 
  faced  on,  or  belonging  to  a  hand  faced  on,  the
  table, including a card from dummy’s hand.


But a clearer view is given in In bridge, is there a penalty if dummy incorrectly lays down his hand? (not quite a duplicate but very close).