Yeah, I went through that whole thing too. My best friend asked me out, completely out of the blue. I did the same thing as you, said I wasn't interested and didn't think of him that way. A few years later and I find things didn't work out quite how I imagined, we are getting married next summer :p. So before I get into the actual issue you have, let me just advise you not to be open minded to the future, some things happen that you never would have imagined.

Now, as for your friend's friend. I'm not going to tell you that you should or shouldn't try going out with him, I'm only going to point out the things that I'm sure you obviously know. You liking this guy is VERY complicated, and depending on how you handle it, you could be damaging multiple friendships. If it wasn't for your best friend liking you, I would say there's no problem with going after his friend. Go nuts.

But since he does like you, he will, at a minimum, see it as a betrayal for you to go after his friend. This probably seems really unfair, but it's just the way his mind is working right now. The two of you might have made an agreement to not talk about this anymore, but I damned well guarantee that he doesn't like you any less now than he did then. It's hard to say exactly how he would handle it, that depends a lot on his personality. But even if he puts a happy face on and accepts that the two of you are together, he WILL be hurt by it.

You need to consider whether or not you like this guy enough to endure the potential damage. That's what it really comes down to. You could lose your friend of this, he could lose his friend over this, is the risk worthwhile?

P.S. I'm going to direct my boyfriend to this in the morning, he will have something to say about the state of mind of your friend. Probably a lot to say

Well just talk with his best friend .... if he REALLY likes you ... than go for it , fight ! . Forget about this other guy you dont like ... he introduced you ? so what ?? You DO NOT owe him anything.