Relationship advice needed, what would you do if it was you?

Last week, on my way home I stopped by an eatery to grab some snacks. As I was about to leave, I turned and my eyes collided with my bestfriend's(girl) fiance. The idiot was having a romantic dinner with another girl. We both saw each other, so no need for pretence. I greeted him and he responded.
On my way out of the eatery, while wondering how I will position my mouth to tell my bestie that the man he is about to marry is a dog. The guy ran after me.
"Please don't tell blossom what you just saw, please, it was the devil's handiwork, it's not what you are thinking, I can explain..." He kept on begging.
As a good friend, I am meant to tell her but hell no, I know the countless amount of heartbreaks she passed through, before falling for this dude and she is so sure he is the right guy for her.
I just don't want to be the one to spoil her joy, Less I forget, their wedding is in less than a month time... Just imagine the plans, wedding invitations and the rest.
I just can't be the person to spoil all that... Its easier to pretend like it never happened.
I am really confused at the moment.
What would you do, if you were in my shoes? will you mind your business like I am doing or open up, even as you know it might lead to immediate termination of the wedding to be?