Four characters are in the midst of a very ambiguous and dramatic situation, but all of them sing their own respective text and music lines, which intertwine and create beautiful harmonies. The Duke and Maddalena interact, but Gilda and Rigoletto sing for themselves. The drama is frozen in a single moment. On the other hand, in a spoken play, the drama moves forward in such scenes. It isn't restrained by music.
She means that whenever they find out the truth they will turn against what they thought was right and become their allies. I would hope she became right. I don't actually know what this play is.
