The Catholic in this situation should grow a backbone! And this comes from the other half of the relationship… the person who got told there would be no wedding if it were not in the Church. My husband wanted to be with me more than anything in the world… EXCEPT his good standing as a Catholic. I was a fallen-away Catholic at the time, so I did not have to deal with conversion or anything like that… but for many people, returning to a faith they have abandoned is almost as much work.
If being Catholic and marrying in the Church is important to you, then DO NOT try to find loopholes or ways around the rules. You will regret it in the end. Just do it right from the beginning. If you tell the person you want to marry that you will only marry in the Church, and they refuse, well, then, you have found out that the person is not a good match for you… haven’t you?