Full and final Settalment

Hi Specialists,
Employee resigned from organization on instruction of managment with one months notice period but as mentioned in his confirmation letter the notice period was 3 monts on either sides, now he claiming his 2 monts notice period payment employer is not responding on the issue, plz Advice the further process.

Was the instruction for resignation in writing?
Strictly speaking he should have been given notice as per agreement.
One thing is certain company will not pay unless one takes legal approach which may be costlier than 2 months pay.