Start with a global variable called 'CustomerCount'.
All of your customers that come in should have a variable called 'Number' and another called 'Seated'.
When a customer is spawned you attach a the Customer Count number to it and add 1 to the variable (so each customer has a unique number).
When that customer sits down make the 'Seated' Variable 'Yes' or '1' to mark him/her as ready to order.
Choose your way of using a timer, timer behaviour, dt, every X seconds etc and then pick a random customer like this (these are the conditions).
Pick all Customer object (pick all is from the system menu)
Customer object is seated (Click on the object and then compare the variable)
Pick a random Customer object (pick random again from the system menu)
(The above picks all the customers, then refines the search to only customers that are seated and then picks a random one of those to call for the order).
The add the action to the conditions:
Customer spawn object (Order bubble) at image point over the head.