The game does not provide guidelines for this.
D&D 3.5 is not a "life simulator". It is a game whose mechanics largely involve abstracting combat against fantasy monsters. How a character handles the digestion of magical food is a detail the game simply does not address. In fact, the game doesn't seem to consider food an important topic in general, it often being handwaved away. Digestion is way below the simulation threshold - unless it has to do with fighting monsters.
Nevertheless, you are asking a question about a situation that can come up. People can and always will come up with questions the game can't not address, given the open-ended nature of RPGs.
In D&D, such questions are mostly answered by the DM. The DM makes some kind of ruling, and the game continues. If you are the DM in this situation, I suggest you go with whatever ruling that you think is...
- most likely to improve fun at the table right now (is it funny to have a Wizard with a suddenly-rumbling stomach?)
- most likely to improve the story being told (could a Wizard dying of magically-induced malnutrition be a good plot point?)
most likely to enrich/improve the players' experience of the game world (does it being possible for any 1st level Psion to create food from nothing have an odd effect on the setting?)
... and so on. In any case, you're going to have to make a decision for yourself. The game doesn't help you.
Generally, I think you're on the right track to try to enforce some mechanical consequence to the choice of lifestyle.
I think the right choice is to use the second option more often than not. In specific circumstances (especially in social interactions), certain kinds of lifestyles should grant advantage or institute disadvantage. For instance, if you're a stable sleeper, and have to go in front of the Lord of the Town, you're likely to have disadvantage on all your checks. But the inverse is also true, if you're a hotel dormer, and don't make allowances, you're going to have disadvantage with the slumlords etc.
Again, I would make sure there are opportunities for him to...essentially pay...to remove some of this. For instance, if he's been slumming it for a few weeks, he could go to a bath house, pay a GP or two to get a good bath and get cleaned up. I might still harm him if rumors are floating about, but at least he'd look like he was supposed to be there. In other words, it shouldn't be a mechanical penalty that is permanent or robs the player of his agency. Just one that he has to work around occasionally.
As far as awarding inspiration, this should be very much tied to the BIFT (Bonds Ideals Flaws Traits) of each character rather than anything specific to their lifestyle. However, if they are (for good reason) entering a lifestyle that is in character, but conveys some kind of situation disadvantage (for instance, a Hermit staying in the woods, even if it means he misses something important in town).
Basically, inspiration is a reward for taking consequences for staying in character. So if your PC, within their character, decides to slum it, but then has disadvantage with the town noble the next day. That's actually cause to award inspiration, rather than the converse (rewarding them with inspiration for living an out of character lifestyle).
Best Answer
Like the other answer says, there's no system for this in the pathfinder rules. So here are a few house rules you could use. I would start by inverting the rules for starvation and thirst.
As you get dehydrated, you make successive constitution checks and start taking non-lethal, and then lethal damage every hour. Further, characters who are starving or dehydrated become fatigued. So, being well-fed should invert these effects in some way.
Here's an example status effect I've come up with that could implement this:
And also a potential implementation of Profession (Cooking)
And finally, some sample items