I went out with a guy (let's call him Bill) after he split up with a good friend (let's call her Amy). We took some time to pluck up the courage to tell her, as she had basically come to me crying about what an idiot he was when they split up. That was quite interesting.
Friends are more important, I think. Even though our relationship was a success, that was only because Amy knew that my personality was better matched to him (her weak personality was being squished by his strong one. My strong one fought back).
So yeah...if it will alienate your friends if you date him, don't. If they will still be there for you, do.