European rugby final- ticket sales

Got a mate who is Leicester tigers fan . They are through to some final or other on Friday 21st May and have already bought tickets for Twickenham. How does that work when they don't know it will be possible to attend? Presume it's been done on a presumption which must be good news for us surely - they wouldnt sell them without being in the know? Could we conceivably start the process now we know we will be at home 1st leg?