I believe magic is not yet warranted. There is lots of stuff to strengthen the love between a couple but that can be used anytime...
If the upcoming marriage is to be a solid marriage the TWO of them have to remove the meddler TOGETHER, or remove themselves from the meddler's company at all costs. If her husband to be is not totally on the same page as her SHE needs to make sure she needs to extend the engagement or postpone the marriage indefinitley.
My heart goes out to you. There are some seriously defective people out there and if you confront this person things could get uglier. I hope the two of you can find a way to avoid then eliminate her from your social circle.
If things get worse, please feel call me at the shop and remind me who you are and of this post on our ORACLE.
Love, Lady Mimi