It sounds a bit strange, but apparently Chihuahua’s can be susceptible to low blood sugar (AKA hypoglycemia). If the dog is warm, safe and dry, but is still shaking, try feeding him smaller meals but more regularly instead of one big bowl in the morning and another at night, try 4-5 small bowlfuls throughout the day. If this makes a difference, then make sure you pass this information on to the owner when they get back (they may not have been aware of the problem).