More than being oblivious, the problem with Dev and most characters this season has been that none have learnt from their past mistakes. Dev despite having experience of his hiding things from his wife , has been catastrophic for his relationship , will not share the truth with his wife .
He just doesn't understand where he needs to draw the lines , be it his mom , or now Sanjana. He just lacks perceptiveness and foresight . The best way in this situation would have been to tell Sanjana in no uncertain terms that she's toeing lines and serve her with the termination, right there . You don't think about her livelihood when the woman isn't worried about her own , breaching professional ethics by proposing to boss right in the workplace.