I think the situation is that your wife's employer has offered her another job so under the law they've done what is reasonable to try and protect her continued employment. The law does not necessarily say that the alternative job has to be the same working hours as the old one and if your wife cannot accept it, the company can legitimately make her redundant. However, your wife does have the right to ask the company for more flexible or different working hours to accommodate her new situation but the company is not obliged to offer them, so her best option is to ask for a meeting with her employers, explain her situation and ask whether the working hours can be changed.
A good employer and/or boss would have been liaising with your wife during her maternith leave to keep her in touch with what has been happening in her absence. That unfortunately hasn't happened and if a notice about changed paydday dates went out to all employees, your wife should have been included in the circulation, even though she was on maternity leave.