If it was casual work as I assume it was, just tell the job centre she has not worked since the last claim. Don't be scared of phoning the job centre and asking for advice , they would far rather claimants told them the full story , they will not penalise her for being honest. They may well contact the people she worked for and confirm her working hours.
My son was in the same situation, some times he had some casual work for a few hours, the job centre just accepted what he told them about the times he had worked and payed accordingly.