There were many during prophet times who converted to Islam because of marriage liability (as other spouse was already converted and they wanted to keep the marriage intact)> there is no obligation in Islam as to why you are converting as long as you are converting. Please watch before you pass such fatwas
TLK is right.....there were many that converted in the times of our prophet....however....those that did convert followed through on practicing the religion too. Without a doubt those were much better times.
The problem with converting for the sake of marriage in today's times is that many take the initial step but then fail to follow up. Often these folks don't even learn how to do namaz....they simply follow along when in a jamat.
It is the rare minority that truly accepts Islam into their hearts and lives. Let's hope that the bride-to-be in this scenario will do so insha-Allah.