How he comes into the story plays out slightly different in book one. Unlike in the movie, John Ambrose was actually friends with both Peter K and Lara Jean when they were in middle school, but he moved away and they all lost touch.
While Lara Jean is fake-dating Peter, she goes to a Model UN session that John Ambrose is at because she's so curious about him (and whether he got her letter). She walks up to him, but is too embarrassed to say anything and runs away.
In the second book, we learn that John Ambrose actually recognised Lara Jean, and it causes him to write to her. The two of them become penpals.
I won't say any more, because I don't want to spoil it. But it seems like even with the changes that have already happened in the movie version (ie Peter and John Ambrose don't know each other, and John Ambrose shows up on Lara Jean's doorstep rather than writing to her), it's likely that the broad strokes of the story – and John Ambrose's part in it – will remain the same if a sequel happens.