Leaves turn yellow and looks like it will die.
This could be watering issues. Grapefruit Trees need water, especially during dry spells, but too much water will also cause the leaves to yellow and drop off.
Adjust watering and make sure that the soil is properly draining.
A young Grapefruit Tree may have some yellowing of leaves in the Spring, but generally will correct itself.
Nutrient deficiencies can cause yellowing of leaves. An application of micronutrient sprays can help.
Excess fertilizer may cause yellowing of leaves.
Check for other signs of disease or insects.
Sun Scorch is also possible, if the yellowing of leaves is on the west or south side, where the sun is strongest this can be the cause.