If the food was cooked to proper temperature, it would take a minimum of four hours for the food to develop enough bacteria to make someone that sick. It is a ServSafe rule to throw out any cooked food product after four hours of being outside the temp zone.
If the caterer cooked to temp, delivered and then the customer didn't keep it heated for four to five hours, that is on the customer. How dumb are these people to just let cooked chicken set for up to five hours and then eat it? Regardless of anything else, they share some of the blame.
The other option is that the caterer didn't cook or hold to temp, delivered and then the customer let it sit for 5 hours, which would definitely make 30 people violently ill.
The only way to cover yourself on drop off catering is to take a temp reading when delivered and then have the customer sign off on reheating and storage instructions. If you do that, you are always covered legally.