I dunno, unless some criminal stuff is involved, I feel like it is not the business of an employer to probe why a candidate left their previous job in six months. Maybe it is a red flag if their entire resume is filled with 6 month stints, if it is just one job, why bother?
Also, the candidate can say something like "the contract ended" or something generic. What is the point of talking about the shortcomings of their previous job? The interviewer and the interviewee should be interested in what they can do for each other, no?