When or if redundancies come along they only have to look at 23 hrs not 30.
Unless it is also in her contract about avarage hour then she may only be entitled to 23hrs worth of holiday (pro rata). In other words if she goes on holiday she will be paid 23 hrs and not 30.
Perhaps she should be asking for overtime payments for the extra 7hrs. They may change their mind then lol
However I am not sure of the legal standing as some forms of payment/benefit is worked out on average pay over a given time.