He was also pretty tipsy during the treble celebrations.
Pique was also drunk as s... during treble celebrations and noone seemed to care about that.
Oh, and Messi during a celebration of Pep's treble (I think) too. And Iniesta and so on and on. They all deserved to celebrate for a day or two after a long and successful season. I'd be more worried if they would be partying while we wouldn't win anything.
Personally I also don't mind if players drink something during their off-days or hollidays. As long as they know their limit and where they are (if they are in public they should behave themselves). I don't think Neymar has really crossed any line so far to be concerned about him. But on the other hand he's a young Brazilian superstar and we have some bad past experience with them so the club (mostly indirectly through his more experienced team mates) should keep an eye on him.