I'm going to ask out a girl who is a best friend of a girl who had a crush on me
That sounds complicated. Do you think things will work out?
It would be nice if they did.
It is good to be hopeful that things will work out when you ask her but it probably won't. Her friend will be upset and it could lead to the end of their friendship.
Fuck, now I really regret it.
Yeah, I really believe this is something to be reconsidered. Is it really worth breaking up their friendship by asking her out? How about just finding someone else or giving the one who has a crush on you a chance, there are plenty of fish in the sea.