What did the Duke do to help Shylock?

Not a dang thing. He went out of his way not only to thwart Shylock's revenge, but to take his money and give it to his useless son-in-law, and finally to ensure that Shylock's soul was damned, all on the basis that because Shylock was Jewish he wasn't entitled to the benefit of the law like the Christians were.

The Duke didn't do this earlier because he was too dim to figure it out, until Portia told him what to do.