Whey powder is made from drying Whey or liquid plasma. This is the liquid that remains behind in the cheese making process after milk has been curdled by the addition of rennet and strained. Whey made using rennet derived from pig or calf which has not been slaughtered islamically will not be Halal. Whey made using rennet derived from fungi, plants, micro organisms will be Halal.
One must check with the manufacturer of the whey powder to find out the source of rennet.