I was at that practice. they were not buddies. I think there may even have been a line call issue on Grigor's serve. Grigor went back to the baseline saying, "I'm not going to say anything" then proceeds to serve an ace after which he said "that's my response". could have been banter, I suppose, but JJ started serving really hard and grunting on each shot. did not look lighthearted.
Well they look like buddies in this pic, so I guess maybe banter, considering JJ also started grunting loud at boodles challenge for fun vs Grigor.