I'm going to ask out an 'ex' of my friend
Why would you ask out a friend's ex?
The ex was one of my first loves and I want to prove my dumb friends wrong by having a successful relationship with her.
It's an unspoken rule not to date ex's of your friends and it's the same in this case. It may be very painful for your friend to have to deal with his ex gf dating his friend right in his face.
I'm a man and brave.
I can see that you are confident in your ability to make this relationship work, or that you are prepared to deal with the grief if it doesn't.  However, this is not just a matter of your own feelings, and you need to think about how your friend will feel.  Your relationship with this woman already ended in the past, as did his, and it will just be too hard for him to see her around you.