Many developers will ask for a testing or sandbox account so that they can test or work with the API during development.
To do this just setup an additional Client on your account.
e.g. Client API Test
Once the Client has been created you'll just need to provide them with a Client User account which they will be able to use to generate an API key.
The developer can then create products and orders via the API without affecting any of the clients day to day order activity.
Once development and testing is complete all you would then need to do is provide them with a Client User account against the real live client and they will be good to go.