The dog set upon the cat. (began to attack the cat)
The person set the dog upon the cat. (caused the dog to begin to attack the cat)
The second meaning could make sense in the original sentence with John being the attacker, a date (fruit) being the target of attack, and an unspecified party being the one who caused John to attack. In this case there is a past-tense passive with "was" + the past participle of the verb (like "was liked", "was seen", "was taken"), but the past participle of "set upon" is identical to the present form, so "was set upon" means either "was attacked" or was caused to attack. But only the second meaning is plausible when followed by "a date", as opposed to "by a date".
"John was set upon a date" → John was caused to attack a date
"John was set upon by a date" → John was attacked by a date
This reminds me that phrasal verbs really are one of the trickiest things in English. Some of my non-native speaker friends have several books just about this topic, because it's so subtle and pervasive in English.