。
Never betray your friend's trust.
Literal
Friend-of trust [object-を] never betray-[negative-imperative-な].
決して ('never, by no means') is an emphatic adverb always paired with a negative predicate. Here it pairs with the rough negative imperative ~な. 裏切る ('to betray') is the verb; the object is whatever is betrayed (trust, friend, country). A firm moral prescription.