The “sabotaging” of a candidate won’t happen very often if the responsibility for providing references is placed on the job seeker! Why would any job seeker ask a coworker that he didn’t really trust to serve as a reference? It makes no sense to do that.
Nevertheless, it does occasionally happen; and that’s another reason why a prospective employer should check at least three references. In my 36+ years of experience in the reference checking business, I’ve never seen a single case where all three references were trying to sabotage a good candidate for employment. If two references rave about the candidate’s overall job performance and the third one essentially says the candidate was a bum, common sense says that something might be amiss!
The solution? Simply ask the candidate to provide a couple of additional references. Then, hopefully, the opportunity will come to ask one of those new references casually why reference so-and-so’s comments about the candidate were so negative. If additional references cannot shed any light on the negative comments, additional confirmation about the candidate’s performance – one way or the other – should be forthcoming from the additional references.
The other thing to consider doing is asking the candidate about the sort of relationship he had with the reference who made the negative comments. Doing that should provide additional insight, if any is still needed, into why the negative comments were made.