Negligence Issue - Customer gets injured badly by my staff's mistake.. and wanted to sue us..

During one night, we had an incident; it was always as a normal day, lots of customers came and most of them did ordered alcoholic drinks till they get drunk. Though it is not against the law to get drunk, I clearly am awared that if they attacked someone during that state / causes any damages without their own consent, they won't be liable to pay for any of the damages that they did. So, one night. There is one customer who ordered too much drinks, though we are not sure if he was in a drunk state or not. During his 3 pins, he dropped 1 bottle to the floor and causes a wet slipper area, my manager did called one of our staffs to take care of the mess by cleaning up the floor. Probably because of the busy schedule, that staff doesn't wipe it out cleanly and still left some spills on the floor which causes the area still slippery. A few moments later after he finished his pints, he wanted to ordered up some more but fell through that area and broke his arm. Whose the one to blame ?