And just as an addendum, and possibly to clear up a slight bit of ambiguity from the earlier part of my post:

Scenario 1: I am involved with a girl or else have made my feelings for a girl known to my friends, but one of my friends also begins to get close to her, I'd expect him to speak to me about it. I'd then have the option to either say 'Cool man, go for it' in which case I have nailed my colours. Or I also have the option to say 'No man, back off please', in which case said friend should respect that decision and find another girl to chase - of which there are many.

Scenario 2: One of my friends likes a girl, is with a girl, or has else made his feelings and intentions clear, but I decide that I also like the girl. I should feel close enough to that friend, and be man enough, to speak to that friend and explain my feelings. If that friend says he doesn't mind if I take a shot, he has made his position clear and cannot then feel harshly done by if I make an approach. However, if that friend tells me to stay clear, I respect this decision, and back off.

As a real life example, one of my friends (J) used to like a girl called Emma. She was pretty, and he'd done well to get her. They fooled around for a while. Things eased off after a few weeks. Another of my friends (D) - a genuinely good feller, and among my best friends in the world - got close to her a few weeks later. D asked J if it was ok to proceed, and J said it was cool. So he did.

After they'd been messing around for a few weeks, J confided in me that he wasn't happy with D. Similarly, D confided in me that he felt slightly harsh towards J. In both cases, my advice was the same. J had the opportunity to tell D not to get too close, and he did not take it. Therefore, Emma became fair game, and J could not complain for the consequences.

Then again, a few weeks ago, another of my friends (J - a different J) kissed a girl that another of my friends (D - a different D. Why are all my friends Js or Ds?) had a thing for. He told me one night, and asked me to keep it to myself, as he did not wish to tell D. Obviously I didn't say anything - its not my place to gossip. But I gave J a very hard time for it that night, and told him I thought he was a shit for doing it.