I just tried it, and it was working fine for me Sarge, so not sure what he's doing differently?
I went to the Ticket Site, then clicked the tab for the Swansea game, picked a random seat that was showing as available, and all was as it should be. Just a thought though, only a few blocks are open, so you might want to check that he's not trying to buy in one of the blocks that isn't open, as the SSL, fence end, where you normally sit is not available for this game.
Having said all that, there should be no problem buying on the night.