You may be approaching this issue from the wrong angle. If you sincerely believe that God has directed these two people not to be together, then why would you want to change his mind? Surely God knows what is best for these two people.
It may be worth asking why you feel God is directing these two people in this way. Is this relationship unhealthy or leading one or other person astray? In that case then maybe if these two changed whatever was causing the problem then perhaps the situation would be different.
The important question is, are you willing to let yourself be guided by God?