“The Good Thief’s [criminal] rebuking of the other’s mocking was a work, a work born from his faith in Christ.”
True, as any human response can be considered as a “work” in the strictest sense, but Biblically it is an outwardly confessional faith which testifies of the possession of saving faith. The key aspect is that it is not on account of any merit of such works that one is justified by, but by imputed righteousness, appropriated by God-given faith, which rests in Christ as his Savior and follows Him as his Lord. To God be all the glory.
Now, works themselves do not merit grace - they, along with faith, are the means of accepting God's gift. Faith and works do not exist independently - they compliment each other! Why else would Jesus command actions of his followers and not mere faith?