I've worked on buses. The company, and the driver, could face potential lawsuit charges.
A DOT law states that any bus equipped with an emergency exit - school bus, travel bus, tour bus, etc. - has to have a warning buzzer or some other instrumentation that shows to the driver that an emergency exit is opened, installed and operational so it sounds or flashes when any emergency escape is opened. There may not necessarily be a warning that states that to the public themselves (typically it's basic "emergency exit" markings), but the driver is at all times supposed to be aware of such a system and check it's operation on his/her pre-checks before operating the vehicle. All emergency exit warning signals are operated by tiny micro switches. Some of them are literal switches, while others are simple push-buttons like on car doors that depress when the exit is closed. When the circuit is broken (exit opened), either a buzzer goes off or the warning light comes on.
If the man managed to open an escape hatch, either a very loud buzzing should have been heard or a "dummy" light should have illuminated on the dash. If neither of these occurred, the driver did not complete the pre-checks of the bus correctly and so didn't report the fault.
This however is dependent on what "party bus" means. That could have been a simple van owned by a private company. In that case, I'm not sure exactly how that would be handled, as I'm not sure if they're covered by this ruling, but I do know all school and commercial buses are part of this DOT law.
[This message has been edited by Fiero84Freak (edited 06-21-2011).]