The one dialogue from tonight's episode that did bear the sole definition of Zain and Aliya's friendship was, "Jab Dost Itna Acha Ho To Ishq Ki Kya Zaroorat". Their friendship is not only the foundation of their relation but also turned out to be the reason for Aliya's nod to their scattered relationship.
The link to the whole article is below :)
http://www.fuzionproductions.com/beintehaa-important-friends-lovers/