First and foremost he needs to apologies to her family for the amount of misery he has given to them. He shouldnt forget he is a brother of 3 sisters . What if some one had done this to his sisters. So he should be standing in the shoes of her parents and thinking...He should be able to win their trust and confidence first.
Then try to speak to Sona...and give her time to understand what went wrong between both of them. Its not just Dev , I feel Sona was equally not ready for a man like Dev. Even though she told him , she was ready ...I dont think she was prepared for this. In a relationship , it isnt nice to blame one alone...both are a equally responsible when something goes wrong because they are a team. When one looses the other automatically looses . There are times when you have to give to the likes of the other...so its more of give and take...I feel they should give time and space to get the trust and confidence back in their relationship.
Then they can think of getting married. Till then keep the relationship absolutely to the level of best friends.