I agree. If she is definitely done with Catholicism and is switching to a different tradition, this really isn’t such an issue. However, if she is just non-practicing, it can cause her a headache if she ever returns to the Church.
Take it from me – I was married over two years ago in my wife’s Lutheran Church. I was a non-practicing Catholic at the time and frankly wasn’t even aware that I was bound to get married by a priest or deacon. Several months ago, I returned to the Church and a short while later I stumbled across the section in Canon Law that talks about invalid marriage and convalidation before returning to the Eucharist, etc.
You may just want to inform her of this. If I was aware, EVEN being a non-practicing Catholic, I probably would have sought dispensation. Don’t refuse to walk her up the aisle, just inform her that if she ever does return to the RCC, this is what can happen. If she still decides to marry without dispensation/anullment, you at least did your part and she made an informed decision. Then you can walk her up the aisle with a clear conscience.