Earlier today, a group of us were playing Scrabble. One of us had to leave early due to something external. What happens to that player's tiles?
While there are no explicit rules in Scrabble to handle this, it's pretty straight forward to do this in Scrabble with minimal disruption. I've done this on several occasions.
Leave any tiles on the board where they are. Even if you could remember which tiles they played, you wouldn't want to. Tiles that are played are essentially communal property, as any player can play off of them. Also, removing played tiles from the board would be potentially disruptive to the next few plays, where leaving them has no ill side-effects.
As long as there are still tiles left in the bag, return their rack of tiles to the bag without looking at them. The game is predicated around their being a particular set of tiles in circulation. Taking these tiles permanently out of circulation is bad for anyone tracking tiles, and keeping them in circulation doesn't hurt. (Note that due to a lost challenge, some of these tiles may have been revealed, but in that case, they've been revealed to all players, so there isn't really an advantage to any player.)
If there are no tiles left in the bag (it's endgame), keep their rack out of play and face down. At this point, players are no longer drawing replacement tiles, and so suddenly introducing replacements would have a profound disruptive effect on whoever went next. You may want to even do this if there are very few tiles left in the bag; that's up to the table.