So many possibilities with this one really...
1: Translation error/Kishi wrote wrong - Doubt it.
2: Kiba was making a joke of some kind - Doubt it.
3: Kiba was referring to the Chunin Exam where Naruto beat him with a fart, just that he told the girl that Naruto just quit in that fight against him (thus, Kiba claims to have beaten the 7th Hokage in a battle to impress the woman) - Most likely.