" But why did he refuse? It won't look good if everyone goes except Aaira." These are Advaita's words to Ansh on phone when he tells her that Aaira will not go with them.

Well, it is a surprise for Advaita that they are going there to celebrate her birthday but it is not a surprise as to where they are going, because first of all Advaita herself knows that her birthday is near and secondly, Advaita cannot go the entire journey blindfolded, so it was necessary to tell her where they were planning to go.