Thanks for your enquiry.
Generally, If a pension scheme makes a mistake, it's normally the case that you do not have a right to the sums given to you in error. The basic principle is you are only entitled to your correct benefits, and not to figures quoted by mistake.
However, if because of the mistake you made decisions or spent money, and you would not have done this otherwise, compensation may be payable to make good the loss you've suffered.
If the mistake that led to an overpayment was made more than six years ago, it may also be possible that overpayments made more than six years from when you are told of the mistake, do not need to be repaid. This will depend on if it was reasonable for the mistake to have been found earlier than it was.
This is a complex area and I would suggest that you speak to the Pensions Advisory service, which is a free service! They will be able to discuss your Wife's case in full with her. They can be contacted on 0300(###) ###-####
I hope this assists you.
Unless your Wife should have known that she was being paid too much (ie she should have known that the amount was to be halved), then she would not be obliged to repay any overpayment which is over 6 years old.