Who instigated the first time you guys hung out? Who's idea was it?
I'd recommend trying to set things up with them again, flirting with both of them as you guys hang out (more her than your friend), and at any window to talk to the friend alone set up a date / get her number / bust a move if you're really feeling it (your friend goes to the bathroom / you tell the girl you want to stay put for a second).
If your friend doesn't want to fuck you, she'll probably wingman for you a little bit (especially if this girl has told her she's interested in you).
If your friend does want to fuck you, that's another barrier to get over because she might try and cock block out of jealousy.
While I've seen guys get away with asking their friend "hey, what's [girl's] number?" and then texting her to set up a date, more times than not it goes south because now you've directly involved the friend in the dynamic --> more reputation concern of being sexual / monitoring of your guys' current state of affairs. So, while it's doable, I wouldn't really recommend it.
I hear that going to a girls friend to tell her you like her friend is a rookie mistake.
Yea big no-no IMO. Makes it look like you're too afraid to be open about your interest.
Hue
