Nothing literally
The main concern would be dehydration - most people can go a surprisingly long time without eating any food (I’ve been on 48 hour fasts with no issues - I only feel hungry during the normal meal times and that hunger then goes away, the record is over 350 days [granted they initially weighed nearly 400lbs]).
Assuming you mean food and liquids and not things like oxygen? Low blood sugar, probably low blood pressure, starting stages of dehydration, probably feeling lethargic and even nauseous. I’d bet there’s a strong stress correlation in this persons life right now that’s stopping then from taking proper care of themselves. Hopefully they eat and drink something, maybe also take a vitamin with an extra glass of milk or water and a good nap, maintaining one’s health is no joke after your twenties.
I mean you probably wouldn’t feel good, you’ll probably be very dehydrated and your blood sugar will most likely be out of wack, but like in terms of long term issues, you shouldn’t have any if it’s just this one time.
I saw you said this person is nauseous- in my professional experience, not eating can make a lot of people’s nausea worse, so I would recommend this person eat an easy-to-digest snack or small meal
I do pretty regular 48 hour fasts, so 26 isn’t that bad. I typically break my fast with some easily digestible sugars, like a few pieces of candy. This helps keep away nausea and bring my energy back to normal levels quickly.
If they were up for 26h straight, my guess is the nausea is related to that more than the hunger.