When assigning blame, there are a multitude of phrases and synonyms that one could use in place of "was at fault." Some possible options could include "made a mistake," "had an error in judgment," "acted negligently," "committed an offense," or "was the cause of the problem." Other phrasings could include "owed a duty of care," "fell short of expectations," or "failed to meet standards." In each of these instances, the intent is to convey that the individual in question has some degree of responsibility for the negative outcome that resulted. While the specific phrasing may differ, the underlying message remains the same.