Sounds like the cook aide **outgrew their job** almost immediately, i.e., hit a *learning/personal development ceiling*. So, I would suggest as an answer the idiom **to outgrow one's job**, and obvious variants required by the context. 

If you Google **outgrow a job**, you'll get tons of hits -- and advice on how to tell you've **outgrown your job** and what to do about it.