Grapefruit, in season September through April, can be extremely bitter, even when it's at the peak of its season and at its juiciest. Some people douse grapefruit in sugar or an artificial sweetener to cut through the tart acidity; however, a sprinkle of salt, although it seems strange, is an effective way to neutralize any acrid flavor.
Try it out at home by cutting the grapefruit into bite-sized wedges, and then sprinkle it with salt (I used fleur de sel). Keep your salted hand at about a two-foot distance, so the salt disperses evenly. As you eat the grapefruit, you'll be amazed that it tastes like grapefruit but without the bitter aftertaste.