Hi Bengalilover!
In addition to your FIL writing about the future wedding plans, ask him to include information about the ENGAGEMENT ceremony that he attended/planned/paid for, etc. for you and his daughter.

Quoting the date of the ceremony, etc. and listing the names of those in attendance for the ENGAGEMENT (with their signatures under a statement about that ceremony being an engagement party and NOT a wedding)...
If you can't be there with her at the interview, make sure she is prepared for these types of questions and that she is fully ready to stand her ground and NOT sign her name to anything that is NOT the truth. Do your best to prepare her for everything about the steps at the Embassy/Consulate (where will the interview be??) so that she feels confident from the very beginning...
Stay calm yourself, and have the names, faxes, emails, phone numbers of your Congressmen, etc. ready and waiting just in case so that you can bombard them with information as soon as possible in case anything goes wrong. (Just being fully prepared always makes me calmer!)

Ask your spouse to pay close attention to everything that happens/is said in the interview so that you can all keep your information/facts straight when taking any next steps.
Good luck with the process, keep us posted on how everything goes!