In a small saucepan, bring the stock to a boil. Remove from the heat, stir in the couscous, then cover at let sit 5 minutes. Fluff with fork and set aside.
In a small bowl or large measuring cup, whisk together the dressing ingredients: lime juice, honey, olive oil, and salt, or shake all of the ingredients together in a mason jar with a tight-fitting lid.
In a large bowl, combine beans and couscous. Pour dressing over top and stir to combine. Add the avocados, grapefruit, feta, mint, and cilantro. Toss gently. Serve chilled or at room temperature.
Leftover grapefruit salad will keep in the refrigerator for up to 3 days.