Reduced hours before redundancy

Someone I know is being made redundant. However, their hours were fairly recently reduced (less than a year ago).

The company is only offering statutory redundancy and is calculating her redundancy figure as number of years service multiplied by current weekly salary.

If they used number of years service multiplied by her old salary this would be considerably higher.

If they used number of years service prior to reduced hours/salary multiplied by the old salary this would still be considerably higher than what she is now being offered.

She did accept the reduced hours and corresponding reduced pay when this was enforced.

Is this a legal loophole that allows employers to reduce the size of payout, or is she entitled to the amount that she would have received had she not accepted the reduced hours? Certainly seems dodgy - but dodgy doesn't make it illegal.

Thanks for any helpful response - but if anyone can point to an official source of the information I would be very grateful.