What the TE describes sounds like having the golf ball already perfectly placed in front of the hole. Now you better don't get too nervous and shaky while using the golf stick to just give it the right push to put-it-in-the-hole (pun intended).
I'd reply the flirting on a cocky&funny base, but no secrets. You have to maintain the attraction and wait not too long. Best idea would be to implement some compliments about her in the coxäcky and funny way and when things get close to too hot, you'd say something like "Ok, I better stop talking now, elsewise I cannot guarantee for what will happen with a smile and then change the topic.
After having this thing done two times I'd try to arrange a date with your friend and her in a bar. Then you can talk more about sex and attractivness without getting blamed by talking about how hot that girl in the bar looks or ask your friends GF if she thinks that this guy over there is hot and so on...then ask her what she likes about men and so on and if they are still following the conversation, you can say something like that your friend and her are really a hot couple and that you, if you would not know them, would be kind of interested in them.
If they are into it, it'll run from almost alone, but you might need to talk later on about the rules of engagement.