But all these ideas had been dissipated by the great misfortune of the death of Lady Mary's mother.

From all this he was driven to acknowledge to himself that his silence in Italy had been wrong, that he had been weak in allowing himself to be guided by the counsel of the Duchess, and that he had already armed the Duke with one strong argument against him.
He did not doubt but that Mrs.Finn would be opposed to him.

Of course he could not doubt but that all the world would now be opposed to him,--except the girl herself.

He would find no other friend so generous, so romantic, so unworldly as the Duchess had been.

It was clear to him that Lady Mary had told the story of her engagement to Mrs.Finn, and that Mrs.Finn had not as yet told it to the Duke.
From this he was justified in regarding Mrs.Finn as the girl's friend.