My sister married my brother's best friend. It was/is sometimes awkward, because my brother sometimes has to be reminded that the marriage is the primary relationship now (rather than the friendship), but for the most part, my brother got over it.
Talk to your friend before you do anything, and if you date his sister, only do so if you REALLY think there is potential for something long-term.
EDIT: how old are you all?
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?