It sounds like there are several problems here (one being the keeper having no interest in caring for this animal) and I am glad you are stepping in. How long has this "friend" had him?
Will/do you have "custody" of him? What species, age (approx) or size, male/female?
Dehydration is your first concern - the wrinkled tail, thin, etc. If he is cold and is not getting proper lighting/heat he cannot digest properly and will not be stimulated to eat in the first place. As was mentioned, if you can get him outside to the sun for a while this will help. Try dripping some warm water on its snout (not in its nostrils or eyes. The idea is to stimulate drinking on its own. You can mist him with water (fill container with hot and it will cool as it hits the air) which will, hopefully, stimulate drinking.
Proper lighting - it needs UVB light (reptisun 5.0 or similar), a basking site (regular 75w bulb in dome is fine - adjust wattage depending on cage size) and a branch he can sit on under the light (be careful about burns).
He may not eat until much of this has been taken care of but dusting some crickets with vitamin and calcium for now will help. If you can not get him to eat try getting some Repta-Aid Insect/Carnivore Emergency Aid - ##73030 (see link). Flukers makes it and you can usually get it at a chain pet store (not expensive). I found it to work well as an appetite stimulant and others on this forum recommend it, too.
Hopefully this animal is not damaged beyond repair.
here are some good sites for you to peruse for info (as well as continuing to ask here):
www.chameleonnews.com/
www.adcham.com/
http://www.chameleonparadise.com/
www.chameleonjournals.com/
all about dehydration
[url]http://www.icomm.ca/dragon/hydrate.htm{/url]
lele
Repta-aid
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0.1 veiled - Luna
0.2 green anoles Jaida & Jetta
0.1 brown anole - Jamaica the j sisters were sent to freedom last month
0.2 house geckos - Gaia & Tia (MIA 
0.2 felines - Kyndra and Líta