RPGs have always handwaved anything not related to their core mechanics. Some games have different core mechanics that they focus on, but you have to handwave the minor stuff or you end up with The Campaign for North Africa And in the case of D&D, the core mechanics are around going through dungeons and getting loot. The economy has been designed around supporting the dungeoneering.
Your player wants to do something that D&D's rules are not well-tuned to handle easily. Those rules and prices are set on the expectation that player characters will be paying an NPC this price, and so the balancing is meant to be "Not out of reach if absolutely necessary, but expensive enough to discourage making a habit of it". The designers feel like their players shouldn't be forming a for-profit dungeon clearing business that hires subcontractors to go do the dungeon crawling for them; the players find their fun in doing the dungeon crawling in person.
As such, the rewards that simply applying this rule would give your party are probably a bit unbalanced.
However, that's no reason the player shouldn't be able to do what he wants. In 3.5, even a level 1 character is one of the best of the best. The type of magic that can be cast quickly enough to be combat-usable (i.e. a single action, in less than 6 seconds) marks them as one of the elite.
So it makes sense that if a character with those healing abilities wanted to, said PC could easily make a living almost anywhere. Such power is rare enough, and in high enough demand, that the player shouldn't even have to roll for success; he should simply roll for how much money his PC makes. (This should be true for basically any PC; even low-level fighters should be able to make easy money giving lessons, and rogues should be able to find legal employment as locksmiths without batting an eye)
I would pick an amount of money that is high enough to be worth doing (you know your game's economy best) but not so high that he is tempted to have his character retire from adventuring and simply generate revenue for the rest of the party.
Because that's what you don't want: In D&D, PCs are supposed to be adventurers. An adventurer who decides "This is a nice place. Good people here, fine wine, and they pay well" and opens up an apothecary isn't an adventurer anymore.
That's fine - it can be a fine way to write a PC out if a player doesn't like their character anymore - but PCs are supposed to be adventurers. A shop owner probably shouldn't really be a player character anymore, unless the other players are fine with this. It will put that PC in a position of having a lot more money than the other PCs, and it will also probably mean large sections of game time dealing with things that pertain only to that PC (meaning, everyone else at the table dicks around with their phones while this one PC plays tabletop Recettear).
TL;DR - Probably those rules are a little too sweet. But I would allow a player to do this, with the caveat that if the PC starts spending too much time running the shop then the player will have to decide if that character is dropping out of the adventuring life altogether. The game won't work if the PCs can get arbitrarily large amounts of cash.