Can a PPL be a safety pilot?
As long as the "Safety Pilot" possesses the qualifications and under the conditions listed below (from FAR 91.109):


  (c) No person may operate a civil aircraft in simulated instrument flight unless—
  
  
    (1) The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown.
    
    (2) The safety pilot has adequate vision forward and to each side of the aircraft, or a competent observer in the aircraft adequately supplements the vision of the safety pilot; and
    
    (3) Except in the case of lighter-than-air aircraft, that aircraft is equipped with fully functioning dual controls. However, simulated instrument flight may be conducted in a single-engine airplane, equipped with a single, functioning, throwover control wheel, in place of fixed, dual controls of the elevator and ailerons, when—
    
    
      (i) The safety pilot has determined that the flight can be conducted safely; and
      
      (ii) The person manipulating the controls has at least a private pilot certificate with appropriate category and class ratings.