The general rule for invitations, whether it is a date or not, is that the person who does the inviting is the person who pays.
In this case, Jon did not invite Sarah to dinner. Therefore, he has no obligation to pay for dinner.
And because Sarah was the one who suggested going out to eat, she technically invited Jon to a meal, which means that had Jon ordered anything other than the free water, Sarah should have paid for it.
For the record, I'm in my 50s. I would never expect a guy to pay for my meal unless there was a specific invitation from him for a date that included the meal. In fact, I've run into so many men who believe that first dates should always be "pay your own way" that I always bring enough money to cover my meal, just in case. Sarah's had a run of good luck and it finally ran out.