Two out of three of my 5th level characters quit for finals week. For the one who still wants to play, how do I figure out what monsters can he handle?
Just don't play when someone is out for a short time
I am going to suggest you a frame challenge: if it is just for a week (or even two weeks), don't play during that time. From my experience, it is better to skip a session or two even if just one of the players cannot attend it, and you have even the majority of your group out for some time.
Playing a session with someone being temporarily out for any reason is generally a very bad idea. If the given player is not coming back again, ever, then their character can become an NPC controlled by GM, and everything is clear. But what if they return after two sessions? Who will control their character's actions? What would happen if their character dies? If the char is controlled by the GM, how would they act if a danger is obvious to the GM who has placed it, but clearly not to the players? If it is controlled by other players, how would you avoid potential metagame issues? If said character doesn't act at all during said sessions, how would you explain someone dropping out of action? How would you compensate for lost valauables such as XP, gold, information?
Finding good answers to those questions might be important if someone being out for a moment is a common issue on your table, but if it is rare, you will save a lot of effort via simply not playing when someone doesn't come.
It may be worth just hanging out with the odd player to watch a movie, play other board games, or set up an entirely separate one-shot targeted at a solo player (of which there are premade adventures available).
Again, this is not a good solution if it is a common issue that someone cannot attend a session or two. In this case you would need to develop a way to solve the related problems I mentioned, e.g. find a game that has a mechanic for players dropping in and out while on move, or appoint someone who would control the actions of missing players' characters, or just avoid playing with people who cannot attend sessions reliably. Again, a completely different issue.