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Looks like the manga changed things from the novel, as per normal. But in the novel, Rogerio actually did reject dancing that last dance with Amy.
This is because in the novel, there were like twenty chapters about the party before the condemnation. Roge's little brother didn't appear: instead, a knight reported to Prince Ernest (it was the day after the condemnation that Will went to that same knight to confirm the details of his brother's transgressions).
And Princess Patricia went in person to confront Rogerio while Amy was dancing with another gentleman, and accused him of having an affair--which he denied. She then asked him if he was in love with Amy, which made him stop and question his feelings and actions, so he was ALREADY feeling like a heel before the condemnation. But he actually remembered that he was engaged to Lydia and shouldn't dance the Final Dance with anyone but his fiancée, so he rejected Amy and stood up against peer pressure from other partygoers who would have forced him to dance with Amy anyhow.
Lydia's fidelity was never questioned in the novel, this manga just made it up.
Note that Rogerio was still an idiot who deserves everything that comes to him, due to carelessness and poor judgement of character. This is just the one occasion where his character is made out to be worse than it is in text.