When my wife went for her back surgery (through her abdomen), we expected her not to be allowed to eat beforehand, but we didn't expect for her to have to take very strong laxatives to empty her bowels and then to swallow barium. Why was that?
If your wife had disc surgery performed through her abdomen, the surgeon had to be sure that he or she doesn't cause any damage to the internal organs.

Fasting before surgery is normal because the stomach needs to be as empty as possible. In this case, however, there's more to the preparation. Because the surgeon is going in through the abdomen, it is essential that the organs, be easily seen and avoided. The best way to make the intestines noticeable, is to fill them with barium and easily detectable. This is why this procedure is necessary.