I believe the citric acid from the grapefruit juice is detrimental to the absorption rate of the whey protein. You should be taking some dextrose or maltodextrin with your whey protein so that you spike your insulin levels for faster whey protien absorption. This is of course for post-workout shake.

Grapefruit juice has certain properties that allow it to inhance the effects of various drugs. In the case of protein I would see no extra benifit by mixing the two together. Choose a juice with a higher glycemic index such as grape juice for your post work out shake.

vitamin c apparently increases absorption of protein (i havnt looked into it a colleague told me about it)

so there is a possible mechanism

however protein shakes are very well absorbed anyway, really its a bit of a waste of time imo