here is how it may have happened ...
Ishwari told lawyer that she got Dev's signature and he needs to get Sona's...
lawyer will tell Dev that he needs some signatures of sonakshi before the marriage for marriage application etc...
Dev will call sona to his office and ask her to sign the forms that lawyer prepared ... Sona doesnt know about prenup (as bijoy never told her)... so she must have already signed them with out reading...
Now when the prenup (signed by both Dev and Sona comes out in future)... it is obvious that Sona will think that Dev knew about this and he cheated her into signing this and that he never believed her that she is not after his money...This time i dont think she will give him a benefit of doubt as he has cheated her way too many times...
That is enough for her to send Divorce notice as she is already on the verge.