Can I ask out the girl that both me and my best friend are interested in?
Does your friend know you are doing this?
I haven't told my best friend about this yet.
Assuming your friend expressed his interest first, it's ungentlemanly to go after her if your friend marked her first. Once he has made his move, you're clear to go, but otherwise, it's quite rude to make a move otherwise.
So if I go ahead and ask her out first my best friend can go dump her? I think he would really dump her if I manage to talk to the girl first.
You should just look for another girlfriend if your friend has already expressed interest in her. People would think you're very rude if they heard that you were doing this.