C.S.
How did this plan get started? Did you say something that wasn't interpreted correctly? Do they need assistance? Show them how they can get that assistance. I take it they are young and may need some direction.
Best to just let it be known as soon as possible and if they have to call the wedding off and wait, then that is what they have to do (hopefully there is no baby on the way). However, good for you for recognizing this isn't something you want.
Just be direct, but short and sweet...hopefully any ill feelings will pass soon.
C.