It is fine if Kalyani believes Moksh is alive and wants to search for him. Yes agreed, he will be a source fresh breath for her in the current situation.
What we all mean is that it would better if she deals it a bit more professionally instead of acting as though everything is fine with her and Malhar's relationship and once they get Moksh they can raise him together and be a happy family.
In her craving for this, she's forgetting that for the society including her, Malhar is a married man.
But if at all they do succeed in finding Moksh, Malhar will not hesitate to let Kalyani keep him as he has sensed her loneliness in the Deshmukh household.