He is going to clap and congratulate sona .. Dev says - " Hamesha ki tarah iss baar bhi aaap hi jeeti ms. Bose... You said you will get engaged soon and you kept your promise.. Now i believe you that i dont have any place in your life anymore... You have a good life MS. Bose..."
Dev will leave from the venue...
Sona will realize that she should not get engaged so fast when her feelings for Dev are still so deep just to prove her point that she is moving on to Dev.. She will talk to rithik and he will support her decision to not go ahead with the engagement.
Dev goes back home and will not know that Sona actually called off her engagement.. He will also stop stalking her online. We will probably see a more practical dev and Ishwari will also see the difference in her son...but will ignore..
Later Dev and Sona may come face to face again and Dev will come to know that sona cancelled her engagement and then he will decide to win her back...
I hope cvs take this approach with out ott drama or killing self respect of female lead