nothing bad will happen in the garden, just rachel bitching arial and end with arial leaving due to embarrassment.
looking at the raws (can't read/understand korean) it seems that they will have their first competition in the banquet that they have to make their own dress (not sure) but instead of rachel doing it herself with a help of marie, it was just marie who did it all and the prince knew as soon as he saw rachel in the banquet that it was marie who made her dress.
the prince asked rachel as they danced who made the dress and she lied saying that it was her and the prince asked her once again and she lied again. after the banquet, the prince went back to his room and seeing marie, he felt a huge wave of relief and comfort and asked for her to play the piano as he sleeps because it helps his insomnia.
i'm not sure if the dress is actually a competition but based on the conversation between the prince and rachel, it seemed like it.