Translation:You have to let him work in peace.
The adverb "rustig" modifies the verb "werken", but it is next to a different verb "laten". Can someone please explain the word order here?
This is a good resource on Dutch word order: http://www.dutchgrammar.com/en/?n=WordOrder.00. It's several pages long and very comprehensive. I think you may find your answer there.
When is it suitable to use the English translation of "laten" for "let" or "leave". In English they are interchangeable. "You have to leave him work peace."