Ask yourself these question.

Are you ready to lose your relationship with your friend whose ex you are about to date or marry? It's not a surprise to see that friendship turns sour after your supposed friend with whim you shared the good, bad and ugly of your relationship chooses to date or marry your ex. Some things are better said than done. You will give your friend the impression that you've always wanted a break up so you could hook up with their now ex. The problem is further complicated if you initiated or suggested ways to help with the breakup. Trust and friendship will be broken for life. No explanation of "I was never attracted to him while you both were dating will suffice".

Love is a choice. It isn't an emotion totally beyond our control. If your friendship with your friend never meant anything to you, then go ahead. If you are sure your friend will do same if position were switched, then go ahead. But if you do value your friendship with your friend, let go. There are over a billion faces in the planet. You will find another man.