It is something related to personal Grooming. Does your HR policy talk of personal grooming anywhere?. Whether this was conveyed to the Doctor at the time of induction?. Not sure.
However if it is the first time, rather than issuing warning letter, call him and talk to him. Make him understand the importance and need of grooming. Still if he continue not to follow then you may issue warning letter.
Always there will be some solution when you talk to employees face to face.
Question is Was the Case of Warning clearly interfered with Hygeiene? If Not going by theFacts, that Disciplinary Action would be deemed Arbitrary Action
Bengaluru , 24.5.19