If you want to argue that he should be paid for the value of his work product, realize that in almost all cases that value is likely negative. Slowing down a developer for a day to work with a candidate is in no way beneficial to a company.

There is always a cost to hiring. When a company decides that the value of having that dev is greater than the cost of hiring them, then it makes sense to hire someone. It may not be an immediate monetary value, but the company decided they needed the dev. So they decided that slowing down a developer for a day to work with a candidate _is_ beneficial to the company.

Plus, like others have said, it really sounds like he was being asked to work on something that was adding value. If a company has decided a particular bug or feature is worth prioritizing then it obviously has value. That means that if a potential candidate comes in and does it, they are adding value (though whether the net value is positive or negative varies case by case).