The correct simple form of the sentence "'Five years have passed since his father died'" is "His father died five years ago". This simpler version conveys the same meaning by directly stating the time of the event, eliminating the need for the more complex present perfect tense and subordinate clause of the original sentence. 
Why this is the correct transformation:
Simple Past Tense:
The original sentence uses a present perfect tense ("have passed") and a present perfect relative clause ("since his father died") to describe a duration leading up to the present. 
"Ago" for Past Events:
The word "ago" is used to specify a point in the past. 
Directness:
The revised sentence, "His father died five years ago," is a straightforward statement in the simple past tense, clearly indicating the time of the event and its completed nature. 
Eliminates Subordinate Clause:
The phrase "since his father died" is replaced by the adverbial phrase "five years ago," making the sentence grammatically simpler.