Depends here is an example.
She went on a career break in May until 1st December. When she returns to work in December she will get weekly tax credit x numbers of weeks out back + some or lots if she was originally paying some tax at the high rate earlier in the year.
If she is going to be out till after Christmas it is probably best to leave it and look for a review of your joint taxes in the new year.
Post a bit more info like her gross income, tax credit, cut off point, date left, date returning and I'll give you a better idea.