Do a mentally retired employee need to serve notice period.

Sir my father is a central govt employee. Now he is suffering from psychiatry & elimination disorder.so we have given application for voluntary retirement. we wanted to give 3 months notice period but from the office it was told to us that they needed 6 months of time because of work load so we had given 6 months notice period for voluntary retirement. Now 3 &half months have already past but now my father is not able to do duty because of mental retardation so sir please say me that can my father now take voluntary retirement without serfing any notice period.