Abhilasha, both Radhika and Dev were at fault.
Dev was at fault more in my opinion because he forced his choice on Radhika. Radhika at that time had no support (not even from her father who didn't want to hear the truth). She was not in a position to walk out on this marriage only with no financial or emotional support. Dev had a right to reject Radhika for his obligation but not to force her to go through the ceremony. Then he blasted her later on for his own guilt and used her feelings for him to hurt her deeply by stating that he hated her. So Radhika became desperate to come to his house.
He also kept quiet despite knowing that Radhika was not married to Rohan. What stopped him from telling the family that Radhika was not married to Rohan? He did it partially because BB was desperate for CB and Padma was distraught. But it was also to protect Radhika deep down maybe because he also wanted her to be around him and also because he didn't want her to come out as a liar in front of the family.
In any case, both are responsible but they are also young and had no proper guidance. They are paying a lot for their mistake while the others who have done worse have gone scot free (Rohan, Maheswari, Virat and even Padma) all because of Dadaji.