Lexan is very subject to damage with most solvents and should only be cleaned with special blends of solvents sold by the same companies that make the spray paints such as GripFlex or Spraylat. The damage is typically delayed in appearing by a day or two and will be in the form of crazing which will ultimately result in the face shattering into a million little pieces.
You should advise your client hat you will not be responsible if the face is damaged in trying to clean it ... and I recommend that you instead upsell him to a new face.