Well if you remember in Neil's firsr post leap scene with khanna Family shweta asked him ' did he came home drunk again', that pretty much indicated that he indeed had bent towards alcohol to come out of his pain, and by that i didnt mean he became out and out alcoholic but yes he did chosse alcohol as a medium of eacape in those miserable 10 years.
But that didnt make him weak or is out of his character coz everyone has their low moments, and Neil is only human.And now when Avni declared that she is going back to kashid, may be that triggered his inner pain and lonliness which by now was subsided by blind anger for Avni, that's why he chose to drink.That is the reason i could make out of this scene but due to saayesha crap consistency has been compromised, that's why viewers are left mildly confused after this Awesome Avneil confrontation and it was kind'a rushed too but still I loved it!