This is a painfull situation.
Would probobly come down to how this third person feels. How close are you and your friend to this person that you both like? If its just someone you've both seen and got a crush on then then frell her/him. But if the person is part of both your friendships then its really problematic.
If it had been me and I had no "history" with the girl/guy then I would most likely be the nice guy and let my friend have a go. But be wary of jealously if you choose such a path. If you cant handle it, it can turn really ugly later down the road.
Gamble isnt fun at times. Take the girl/guy and the relationship might end after a month and then your alone without your friend :P
I heard life sucks, that's why I'm glad I don't have one.