Something is missing from this description. How much communication about this specific conference did they have? You state that they said they usually cover these kinds of conferences, but that doesn't mean they cover every one.

Discussion or even an email about often covering such conference attendance before you're hired, compared to discussing this particular conference and what exactly will be covered, are two different things.

Where I work, they do pay for these things sometimes, but I have to clear it with my employer and make a good case that this is a trip that will have a value for them, too. Did he do that? Or did he just say I want to go to conference X, can I do it week Y?

Choice of hotel, choice of airplane seat type, all of that would also have to be agreed upon by the company in advance. If he is paying for these by himself, they might not know that he chose a first class seat, or a nicer hotel or better room. Or perhaps this conference is just too rich for their blood.