Not offering a particular service is not a crime and also not discrimination. In the example stated, the "particular service" would be "prayer room for Muslims".
The example provided is not optimal, since airport chapels are normally arranged so that people of every religion find what they need (e.g. they provide the Koran as well as the Bible). Of course, it's possible that Dan arrives just when there's a Christian service in the chapel, but then he can just wait for half an hour or so.
In the general case, no private entity is forced to offer a particular service. So even the airport would not be forced to offer a prayer room, let alone a Muslim prayer room. It could however be illegal discrimination if Dan wasn't allowed to enter a Christian chapel because he is Muslim. This is the same as if the waiter in a restaurant denies entry to black people (a serious crime in many countries). But it's of course legal for the waiter to not (specifically) offer food for Muslims (e.g. without pig).