1) First give written complaint to Railway authorities On Pension ( Pension Department), with all Objections, immediately
2) Then Give information to Bank, with same complaint enclosed. so that No one can withdraw the money other than Her.
Just check with Nearest Advocate,who can draft this work for you.
If Son done fraud, then it will attracts Criminal case. May be we have to file Police complaint and Central Admin Tribunal case. This is lengthy case.