Just keep it simple and don't overthink it so much. As Bowen basically said, if someone doesn't show up, they effectively "are there and place last". So if it is a head-to-head match, they lose and the other player basically wins by forfeit. If a 4 player match with 2 advancing, the player that didn't show is 4th. Trying to craft things any differently will just add more issues and inconsistencies, especially with WPPRs. Unless you spell out every scenario beforehand in the rules (NO longer simple), then you may inadvertently create all types of negative perceptions of a lack of objectivity or fairness. If the person decides not to play then they had the prerogative to forfeit, and their lucky opponent gets the benefit. But that is the proper placement of that benefit, not other players that have already lost or players that did not qualify.
Also, if someone forfeits halfway through, they should NOT be dropped to the bottom of the final standings because they already EARNED the wins they achieved. Doing anything else would then grant an UNEARNED benefit to one or more players that lost to that player. Don't try to re-adjust things. It's just the nature of tournaments. It happens and has a simple result - that player loses the match they miss.
