State Retirement Pension - inherited Contracted-Out Deductions (COD) 

I have a very indignant client whose State Pension has reduced on conversion following her husband’s death.  This is because her own Pre-97 Additional Pension has been wiped out by the significant COD she’s inherited from her late husband.  Although on the face of it this seems very unfair, I feel I can explain to my client why this has happened and how presumably the occupational pension she will inherit from her late husband’s work’s pension scheme will effectively compensate.  But I can’t find any clear or specific reference anywhere that I can pass on to her to support what I’m explaining.  Any suggestions gratefully received.