the beautiful dress（in which ）（ Miss jones went to the ball） was borrowed from a friend of hers .
可分为：1.Miss jones went to the ball in the beautiful dress.2.The beautiful dress was borrowed from a friend of hers （in 介词 穿着,在此有实际意义,不能省略）
句意：Miss jones穿着去参加舞会的那件漂亮的裙子是向她的一个朋友借的.
被动语态,句型：主语the beautiful dress+谓语was borrowed +宾语a friend of hers
而Miss jones went to the ball 这是一个成分完整的句子,明显看出是用来修饰主语the beautiful dress的定语从句,可选择用that 或which,但从句意和句子成分看,少了个介词in(穿着),定语从句中that前面不能加介词,故只能用in which.
特别补充：
go to the ball 去参加舞会
ball n.球；舞会 vi.成团块 vt.捏成球形