A co workers daughter had a light rash a day ago, now she has a headache, upset stomach, and rash is still there. What could this be?

Viral exanthem. The most likely cause is a cold or flu virus. If she recently started a new medication it could be that as well. If the symptoms get worse she should see her doctor, but otherwise she should do what you do for any cold or flu - rest in bed, drink plenty of liquids, take tylenol (acetaminophen) as needed, etc.