Refer to elements of the candidate’s interview that can be changed. Stating that, for example, the candidate appeared nervous is unhelpful, as it doesn’t provide any sort of means for the candidate to change this behaviour. Instead it would be better to state aspects that can be amended and to then state how. As an individual who has experience in this field, your first-hand knowledge can be invaluable to a candidate, and your feedback will be considered as coming from an authority.
Other examples of areas to improve might include that the candidate needed to do some more preparation about the University and the strategies, or the department itself, or that they need to be more focused and succinct with their answers to questions.