Faulty formulation: If a sin occurred, Allah must approve of it, because nothing can occur outside His will.
Correct formulation: An event's occurring under divine will is not the same as its being approved by Allah. A sin can take place while remaining prohibited; righteous acts are commanded and pleasing.
Application: A sinner cannot turn a prohibited act into a lawful one by calling it destiny. Whether it occurs concerns divine will; whether it is approved or commanded concerns divine approval and revealed law.