Kovets Teshuvos - HaRav Sh Y Elyashiv
Lactose is not a dairy product.
Rabbi Yosef Shalom Elyashiv [recognised internationally as The Posek Acharon ,the supreme and final Halachic authority in this generation] explains [Kovetz Teshuvos Y”D 73] that lactose, although derived from milk, is not a dairy product with respect to two Halachos:
R Elyashiv also quotes the KeNesses HaGedola 115, who rules that whey is not subject to the decree of Ch Akum. Thus lactose derived from whey which comes from Ch Akum is Kosher without any restriction.