My Friend was Beaten Up at a Bar, Who is Liable?

My friend was at a bar and some guys were raising a huge ruckus getting drunk and acting like a-holes. They were trying to jump people and my buddy ended up getting jumped and got his jaw broke and severly hurt by three guys. They caught the guys, but my question is can he sue the bar, they knew about this and didnt call the police, warn my friend, or do anything to help. Now hes stuck with thousands in medical bills isnt that neglegence on their part the bar was in kansas city, missouri if that helps at all.