Yes Scotland is, I think, alone amongst countries with evolved judicial systems where the uncorroborated evidence of one witness would not support a conviction. The Carloway Report (I think published in 2012) recommended the abolition of this rule but I'm not sure whether the recommendation was acted upon.
Turning to England and Wales, the uncorroborated evidence of a single witness is certainly sufficient to lead to a conviction. The strength and reliability of the evidence would be considered by the CPS before authorising charges. If the matter did go to trial it would be for the Bench of Magistrates or the jury to decide whether such evidence was sufficient to prove the matter beyond reasonable doubt.
Bear in mind, Ben, that if it did go to court your friend does not have to prove that he did not do it. The onus is on the prosecution to prove that he did.