I'm going to date a friend's old hook up
Why would you date a friend's old hook up?
I just want to ruin their afternoons together by making PornHub searches together, steal her liquor and make out with her.
Even under normal circumstances, it is not good to date an friend's ex, but it makes it worse if you are doing it to ruin their relationship. It will also ruin your friendship with him.
They should have thought before they dated. It is also natural that I get all his girls.
It sounds like you are a bad friend. You are trying to move on his ex just to hurt him. You shouldn't do that.