Is it permissible to work in a factory with the intention of gaining experience and mastering the trade secrets for a private project, and then leaving the job after achieving that, and is this considered deception or a permissible method?

There is no objection to your relative working in a flower distillation factory to gain the necessary experience and information for his project, as long as it does not involve cheating or a breach of contract. If his intention is as such, then he must agree on a suitable period of work for gaining experience and fulfill the terms of the agreed-upon contract.