Also encourage her to ask for a detailed reason for failure. I don't mean to add salt to an open wound, but it is virtually impossible to fail an assignment, unless she was writing on a completely wrong question.
The criteria for a 40% mark (pass) for, as an example, University of Edinburgh is:
Work which is adequate but limited. It may include irrelevant material and be
too descriptive and narrative. Some aspects of the question may be answered
competently, but others will be ignored because of omissions in the reading,
factual inaccuracy, difficulty in identifying the key issues and arguments, or
poor style, structure and presentation. In exams, an answer left unfinished
may earn a mark in this range if it gives evidence of the potential to perform
better. In courses involving classical languages the work shows, where
appropriate, a basic understanding of the principal linguistic features of the
subject.
A Marginal Fail (30-39%) is:
Work which is poorly structured and of very limited relevance to the question.
It may be distinguished by a lack of supporting evidence, misunderstandings,
a failure to address the question asked, substantial generalisation and the
lack of any real argument. In courses involving classical languages the work
approaches closely a basic understanding of the linguistic aspects of the
subject but is deficient in important respects.
If your DD has passed her main subject assessments, it would be really odd to even be performing at 'pass' level in her history one, let alone 'marginal fail' 