I'm getting married with my childhood best friend's ex fiance
Is your friend upset about this?
I don't know. They actually needed my help in their wedding planning.
I would think this could be upsetting to your friend. It is not good to betray your friend. You should talk with them about this wedding.
Why should I? It's none of their business.
Agreed that you're free to marry your best friend's ex but ethically it's not right. It would be helpful to let your friend know and be up front and honest with them. Hopefully they would truly understand that you do not always choose who you are going to fall in love with.