Stefan found it hard to believe, noting how the duke barely knew about Cataleya. It would impossible to assume that the duke had a secret rendezvous with the lady. He conducted a short investigation about the daughter of the marquis and honesty, her behavior was sort of spiteful if the poisoning incident was deemed to be true.
He did so when he was puzzled over the enthusiasm of the duke to meet Cataleya in the party like his life depended on it. Imagine the chief knight's shock when Adelard suddenly proposed marriage to her which was out of his character to move rashly.
If he recalled it clearly, even lady Briston appeared to not expect that from coming. Rather than feeling surprise, it would be safe to assume that she appeared rather startled. Maybe it was the reason why she ran away from home after the party. The duke's reputation was definitely not one to miss at all. The lady might be too terrified to even consider a future with someone like the duke.