Offered a smaller voluntary redundancy package because currently on maternity leave

[CambridgeCam]

My wife's employer (civil service) recently conducted a round of voluntary redundancy. She is currently on maternity leave (and will be so for 9 months). All of her team were offered the same end date in July, whilst she was offered an end date of April. The effect of this is that she would receive 25% less money because she would just miss out on another completed year of service and would miss the maternity leave payments. She queried why she was offered a different date to the rest of the team, and she was told (in writing) that it was because she was on maternity leave and therefore as she wasn't currently delivering a service they had offered her an earlier end date. They need the team she is in to keep working at full capacity, so they were offered the later dates. She asked for the same date as her colleagues and was told no. The maths meant that she therefore didn't take the voluntary redundancy despite really wanting to. If the dates had been the same as her colleagues, she definitely would have taken the redundancy. She was also told at the time that there'd be another round for after her maternity, but this is no longer the case... Is this acceptable from a legal perspective? Is there any possible action she can take?