The baroness goes with Maria to her room to help her find something appropriate to wear to the party.
She sneakily strikes up a convo with Maria about how she knows Maria's in love with the captain and she thinks he's in love with her, too.
Maria looks horrified. She says she has to leave (presumably back to the abbey), if what the baroness is saying is true. She starts packing immediately.
The baroness offers to help, and Maria just asks that she not tell the captain what they talked about. No problem.