Some times when you trade with an acquaintance you get a prompt asking if you want to be friends with the other person. Most of the times it does not. It happens randomly too. It could be after the first trade after just trading 1 pokemon or it could happen after the 3rd trade request with that person after trading 10 pokemon.
When and how can I get the friend request to appear?
Best Answer
According to Bulbapedia:
So it seems that 2 Battle/Trades are needed in order to get the prompt.