All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The ruling on whey and whey powder is the same as the ruling on the origin of the milk from which they were taken. The rulings on the kinds of milk differ according to its different kinds. If the milk is from a live animal whose meat we are allowed to eat, like cows and sheep, then such milk is pure without any difference of opinion among the scholars. The milk of a dead animal whose meat we are allowed to eat is impure according to the Maaliki and Shaafi’i Schools of jurisprudence and this is the reliable opinion of the Hanbali School.
As regards the milk of animals that it is agreed upon that they are forbidden for us to eat, then it is impure whether the animals are alive or dead.
As regards the milk of a live human being, it is pure according to the agreement of all the scholars. With regard to the milk of a dead human being, then it is pure according to the Hanafi and Shaafi’i Schools of jurisprudence, and it is the reliable opinion of the Hanbali School.
You may consult the books of Fiqh for more details.
Allaah Knows best.