Early symptoms of hypoglycemia include shakiness and hunger, progressing to confusion, slurred speech, profuse sweating, nausea, fast heart rate, and passing out. Most diabetics can tell when their sugar is going too low.
The first priority is to get the person some high sugar drink or candy. Diet drinks will not work. Apple juice or similar will work well, but high sugar things will not work for long. Afterward the person should have a real meal, with plenty more juice. The biggest immediate danger is that the person will return to a hypoglycemic state. If they have the means to check their sugar, they should do so frequently over the next several hours.
If the patient is passed out, call 911 immediately. The victim will most likely need intravenous dextrose (sugar) at that point. It is usually advisable to lay the patient in the recovery position while awaiting the ambulance.
Hypoglycemia is covered in AHA's First Aid course.