Agree that there are two parts - whether they'd have to sell him a ticket in the future and what happens with the current flight but there may be something in the small print that would allow them to prevent him boarding (though almost certainly subject to suitable compensation).
Your analogy should be changed to: 'whether McDonalds should let you eat a burger AFTER you buy it from them'.
This is simply Easyjet breaking a contract - as long as he's not broken the terms himself. which would be surprising in this case.