>>10 I'm assuming that since you aren't together, the girl in question (I'm going to call her girlA) doesn't want you as anything more as a friend, we therefore have two possible things that can happen:

1) You talk to your friend and stop him going out with her. She likes your friend the same way you like her right, so imagine if girlA accepted to go out with you, and then her best friend (girlB) stopped her because girlB liked you...that would suck right? Well don't do that to girlA then. Also, your friend, who might just have said that he didn't like girlA because he didn't want to worry you, will also end up getting hurt.
1 = GirlA is hurt. Your friend is hurt. You feel bad about hurting everyone.

2) You let them be, girlA is happy (imagine it was you...she must be..really happy about this date), your friend is happy. You can feel good about making everyone so happy. Also sad music will sound better for a little while after, and you'll have a new story to complain about on 4ch.
2 = everyone's happy apart from you, but you get to feel good...sort of.

Seems like a pretty easy choice here...

unless...

3) You go for the big, stupid, pointless romantic "I LOVE YOU GIRLA"..you get a great big adrenaline rush and then she either says "OH GOD ME TOOO" (probably not) or she sits you down and explains that you two together will never happen, and situation 2) happens.

I went with a preemptive 3 once, and I don't regret getting that stuff off my chest..(I didn't go with "I LOVE YOU" or anything, but same basic jist.) 3 won't get you the girl I'd say, but it should finish off your feelings...

This is all assuming that she doesn't know how you feel, of course.

2 is safest.

Sorry about the rather long and confusing post there...