I'm thinking of telling my best friend that she shouldn't be friends with an ex friend of ours
Why don't you want them to be friends?
I don't like my ex friend anymore.
You may not like this ex friend but you can't stop your best friend from being maintaining a friendship with them. Let you best friend be friends with whoever they like.
I want to ruin her life.
Whatever your ex-friend has done to upset you so much, I don't think you should still be trying to ruin her life over it.  It is wrong to seek revenge, you should be the better person and just move on.  Your best friend will understand that you don't want to be together as a group.