One of the reasons Kousei uses to rationalize never making a move on Kaori (and the real reason being his indecisiveness, in my opition) is that she is Watari's girl/romantic interest.
Watari and Kaori are never shown as a couple, or even having anything between them beyond friendship, when they are together onscreen. Kaori doesn't seem to even give Watari the time of the day.
In episode 17, he tells Kousei "For the woman I love, I would drink even muddy water", while trying to usher him into visiting Kaori. His attempt at pushing Kousei into going there seems a perfectly fine wingman behavior, so I am quite confused.
So, there are a couple unknowns here:
In the fist episode, it is clearly stated that Watari has an interest in Kaori (and it is what starts the whole "Friend A" Kousei's self-demeaning thought train). Did they move forward?
Is Watari really wingmanning Kousei when he tells him to go visit her?