Like some of the above posters said, don't do it unless you're very seriously interested in her in a romantic way and don't do it if you just want to have sex with her. And give your buddy a head's up first up and make sure he knows that you really like her a lot and wouldn't do this if you weren't really serious about it.
One of the few relationship regrets I have is that when I was in college, I went out for a couple of months with my best friend's sister, who went to a neighboring college. It started when she asked me to her sorority formal, and I knew I would have stay overnight. I asked my friend about it, he actually was very encouraging because he hated her last 2 boyfriends. I genuinely liked her as a friend, but I was not interested in anything serious and just wanted to have sex (basically with anything that moved, I was soooo horny then). I knew she was really in love with me, which made it even worse. We slept together probably about 10 times. The last time, right before the semester ended before summer break, she told me we had to be careful because she was ovulating. And, of course, that had to be one of the 2 times in my life where the condom broke and I didn't realize it until after I came. Spent an agonizing month wondering if she was pregnant. Luckily, she wasn't, and I took that as a sign to stop being such an idiot. So, I wrote her a letter and broke up with her via snail mail. Classy. My friend told me she was really upset about it for a long time. And, somehow my friend's parents found out she and I had slept together and were furious with me for taking her virginity (which I hadn't). Luckily, my friend and I are still good friends 20 years later, his sister and I hung out at his wedding and had a non-awkward, fun platonic time. But, my conduct on a number of levels was pretty terrible, and it makes me wince with shame all these years later.
