I have a feeling this is going to backfire as this might just prompt Sonakshi to come to the engagement after Bijoy allows as she will understand it is still about Dev's and Ishwari's wishes and they are doing all this because they want her back without knowing what she wants. She will come to show both of them that this engagement really does not affect her.
As for Bijoy looks like he allows Sonakshi to go for the engagement as he knows that probably after seeing Dev getting engaged she will get a closure and will be able to move on in life.
Just my POV, no offence