。
My friend John told me so.
Literal
Friend-[appositive-の] John [subject-が] me [to-に] that-way say-[past-った].
The appositive の again: 友達のジョン ('John, [my] friend') places the relationship in apposition to the name. そう言った ('said so, said that') uses そう as a pro-adverb referring to something just mentioned. Natural everyday reportage.