One of my best friends from high school has an ex-girlfriend that I'm going to ask out.
Have you considered the possibility that this will make your friend upset?
No, he is seeing someone else. And besides, she used to be such a slut, but she's changed a lot.
It's generally not a good idea to be involved with your friend's exes, even if they have already moved on. It will be awkward since you will all have to see each other at various events. Additionally, you shouldn't refer to her, or any woman, as a slut.
Nah, it will be fine. I don't think she still has feelings for him.
Even if there aren't feelings involved between them anymore, asking out the ex of a friend should be avoided because it can create awkward and uncomfortable situations. Doing this will run the risk of damaging your relationship with your friend.