My thoughts are as follows:
HOW - Today's episode #184 leads me to believe that DB will contact him and ask him to return as Nidhi needs her friend and only he can help her.
WHY - To help Nidhi battle and expose Mallu Mata once and for all.
WHEN - Based on SK's tweet, I'm going to go with around June 28th episode or July 2nd episode. He'll show up when Ashutosh won't believe Nidhi that Mallu Mata if faking it.
Please add on your thoughts...Thanks!