Answer:  Old King Cole

Viola is the thief.
Luckily for her, she was able to use her feminine charms to placate the simmering King and avoid a lengthy sojourn in the Dungeon contemplating her defilement.

The fiddlers made eight statements.  The wizard observed that only the thief made two false statements.
The thief could not have stated that another fiddler was innocent as this statement would have been a lie and there would have been a second thief.  So we can absolve Cello and Fidel from any wrong-doing.
As Fidel is not the thief, at least one of his  statements is true.
Fidel lied when he accused Cello of the theft (we have shown above that Cello is innocent), therefore Fidel's second statement is true in declaring Strad's innocence.

Viola lied twice by proclaiming her own innocence and accusing Strad of the theft.