2000 Toyota Avalon Parts employee installed battery backwar

My daughter had her belt break and while the mechanic replaced the belt he also told us the battery was bad. The battery was purchased in Feb '09 so I told her to take it to the closer location of the chain I'd bought it from. From what she and her bf said the parts employee initially put the batter in and connected positive to negative and negative to positive and the car alarm (horn) started going off. He took that battery and said he'd put in the wrong one. He came out with another one and connected it properly. The car started but none of the instrument panels came on or the radio. The windows wouldn't go down etc and she was unable to put the car into reverse. The parts employee says it was his fault and any damage or repair costs will be covered by them but that his manager was out until monday. Her bf had his car there also and they had a banquet to go to so the car is still at the parts store. Could there be fuses blown or is it possible that its much more serious? Is there anything we can try or should we just let the parts store get a mechanic out there to look at it?