In reality the groundschool exams bear little reflection of what the job entails on a day to day basis.
They do however, reflect on an individuals ability to absorb and retain information over a short period of time which is important as extra training costs extra money and those who have struggled may be a training risk and require extra training and therefore cost in the future. One of the main purposes of the interview is to determine whether an applicant will be successful in training. Beyond this the airlines are hiring Captains not First Officers. They need to believe the individual is capable of becoming a Captain.
The fact your daughter has had some interviews already is both good and bad.
The majority of graduates find it extremely difficult to get any kind of interview.
The disappointing/worrying part is that none of these have lead to anything yet.
Were these interviews arranged by Oxford? Has she applied independently to the smaller airlines and operators?
To be successful she will need to be able to explain why the setback occurred, how she overcame it and turn the whole experience into a positive.
Perseverance and confidence are what are going to take her to the next stage.
Good luck!