Enterprise Rent-A-Car Bad Rip-off and Lies!! Fayetteville, Arkansas

My friend and I went to rent a car on 11/12/2011 from Enterprise in Fayetteville, Arkansas. Our plan was I rent the car and I let him drive it. He had to go buy some food for some cultural event at our university. So we get at the store. I rent the car using by Debit card and make mention to one of the agents that  my friend would be driving. He then persuades me to buy their insurance and tells us the car will be fully covered. (He doesn't mention anything about listing my friend as an additional driver; I find out after we return the car). And so I pay a deposit of $250 that includes insurance for two days. I was expecting to get some of the deposit back.  Me and my friend head off and I give him the keys and he drives to Oklahoma; I stay in Fayetteville.

On his way to OK, my friend has an accident. One side mirror is gone, and 3 wheels are damaged. He calls me and I contact the roadside Assistance to help him.  I am told that I violated one of their policies since my friend was not the one who rented the car nor was he listed as an additional driver. They help him regardless to replace a flat tire. My friend continues on to OK makes all the food purchases and returns to Fayetteville Saturday night.

We return the car on Monday 11/14/2011. The customer agent seems happy to see us. He tells us that he heard about my friends accident and is happy that we had paid for the insurance. After a while he tells us that the Insurance will not be able to cover the damages since my friend was not listed as an additional driver. Me and friend remind him that we had told him that my friend was gonna drive and he made no comment about it. He then shows us the contract to "prove" that he had told us. Well what could we do?!! Our word against the contract!!! A little mad at his dishonesty, I listen as he tells me i will hear from Damage Recovery Unit soon about whether I will be charged or not.

Enterprise keeps all of the $250 deposit money.

I get an email one week later stating that the Insurance will not cover the damages considering thefact that I violated one of their policies.
On 12/08/2012 I  receive a letter from Damage Recovery Unit noting the Date of
Loss as 11/12/2011 and a Balance Due of $2704.28.
Damages: $2,256.97,    
Administrative fees: $150,  
Loss of Use:  $71.62.
Diminished of value : $225.69.

 “Our review indicates that you are responsible for the damages to our vehicle. Enclosed please find documentation to support our claim. Please review this information and remit payment in full to the address above” followed by detailed instructions on how to pay.

Some of the damage costs include labor cost and paint cost, and other damages that did not happen as a result of my friends accident.

I need advice: where do I go from here? I know Ignorance should not be an excuse, but the customer agent never mentioned to me that I could add my friend as an additional driver. He let it slide and only told us when he could use the info to rip us off.  

Help!! NEED YOUR ADVICE!!