He can't do that without you explicitly allowing it by GM fiat.
Before we dive in, I wanted to stress an important point: one of your responsibilities as a GM is to use your judgment - a player should never use rules-lawyering to force you into introducing something which you consider game-breaking into the game - if you feel that this is the case, than no matter what the RAW say, you and your group should discuss this offplay and agree not to go to such directions.
In the specific scenario you've described, however, there are also several places where the RAW themselves will deny your player from achieving his "money for nothing" scheme:
1) RAW of 'Aid Another' for skill checks
First, take a look at the Aid Another rules for skill checks:
You can help someone achieve success on a skill check by making the same kind of skill check in a cooperative effort. If you roll a 10 or higher on your check, the character you're helping gets a +2 bonus on his or her check. (You can't take 10 on a skill check to aid another.) In many cases, a character's help won't be beneficial, or only a limited number of characters can help at once.
In cases where the skill restricts who can achieve certain results, such as trying to open a lock using Disable Device, you can't aid another to grant a bonus to a task that your character couldn't achieve alone. The GM might impose further restrictions to aiding another on a case-by-case basis as well.
(All emphasis mine)
Note, that the rules state that:
- The default assumption is that on many cases, assistants won't help (or only a limited number of them do).
- The GM may impose further restrictions on a case-by-case basis.
So, RAW mandates that any case of getting help from Aid Another requires GM's approval.
This unusual case of 100+ assistants definitely qualifies as a "special case" requiring GM discretion.
2) RAW for coercing people through Diplomacy
Let's say your player figures he can ask each assistant to make a craft check of his own instead (or find some other scheme bypassing the Aid Another issue), using diplomacy won't work for his "prolonged slave labor" because:
- While the base DC modifier for "Give lengthy or complicated aid" is just +5, he'll have to continuously ask his assistants for aid, and any "Additional requests" raise the DC by +5 per request. So the DC will become unmanageable very fast.
If he won't compensate his workers they'll start resenting him, leading their attitude down to Indifferent and later Unfriendly (if he plan on working them to death it'll happen much faster...), and since:
Any attitude shift caused through Diplomacy generally lasts for 1d4 hours but can last much longer or shorter depending upon the situation (GM discretion).
He won't be able to improve his slave's attitude long enough for them to willingly work for a week.
Some requests automatically fail if the request goes against the creature’s values or its nature, subject to GM discretion.
It's reasonable for you to declare that asking people to work without compensation for a long period of time is "against their values", and no amount of sweet-talk will make them play along with that.
Having a high Diplomacy is not some kind of domination magic - there are some things you simply can't convince people to do.
3) Other considerations
From a narrative perspective, in a setting with a medieval level of technology, having more than a handful of assistants won't help - you can't really coordinate their actions efficiently, they'll get in each other's way, and the tools, materials and partially completed products won't be standard enough to effectively combine the output of 100 people into one complete product.
Moreover, even if it was possible for your player's PC to create some sort of assembly line arrangement to make mass production feasible, this still goes strongly against the genre. So, unless this is where you as the GM and the entire group of players wish to take your game to, you should probably just agree (off-play, around the table) that you won't - it's a pretty standard part of the social contract of fantasy RPGs anyway...
Finally, as the dark wanderer explains in his answer, doing something along the lines of this would probably be considered evil, and at any rate will be frowned upon by the general public - it will be difficult to hide - and such schemes are a classic act of the protagonists (which PCs are called upon to punish). If that PC goes on with something like that - he is asking for all sorts of trouble, and if he has any measure of common sense is probably aware of that - and you should inform the player of that...
Hope this helps, and goodluck with your game.