My suggestion is to NOT do it.
My now ex-best friend was going out with a girl from our group of friends. She was funny, cute etc.
Long story short, they broke up due to him being a jackass to her. Even as just a friend to the girl, I found it quite disgusting how he treated her. So, they broke up, and I consoled them both. However, a few months before they had broke up, I had started feeling attracted to her, but out of honour and friendship, I never made a move on her, even when they had broken up.
What DID happen, however, is a few months after they'd broken up, we had become really close friends, more so than before, and she was clearly attracted to me just as I was to her. So, we started seeing each other. This was about 5 months after they'd broken up.
My mate was pissed. I could see where he was coming from, but it didn't matter-I figured he'd be man enough to understand that it was simply two people who had feeling for each other, and that he was the cause of their relationship dying anyway. Regardless, he wasn't mature enough to see that, so we stopped being mates (bearing in mind we had known each other since we were 3 years old).
Him going off on a rant shattered our main group, with one friend going with him, another becoming a total scumbag, another becoming someone completely different. 3 years into the relationship, we broke up with each other. It was a mutual thing, and I certainly don't regret it-But those friends who split back then are still apart. Quite sad, really.
So, no - Unless you really see something special about the girl like I did (I.E: Not just a 6-month typical teen romance), don't do it.