The fact that someone else is not following the majority is social support. It enables the naïve participant to be free to follow their own conscience.
The confederate acts as a 'model' of independent behaviour. Their dissent gives rise to more dissent because it shows that the majority is no longer unanimous.
If they did well in an exam it was because they used an excellent textbook, if they failed they might blame it on the textbook or they had bad luck because the questions were hard
Allen and Levine (1971) found that conformity decreased when there was one dissenter in an Asch-type study, even if the dissenter wore thick glasses and said he had difficulty with his vision