If the said girl still has feelings for Dan, then the girl and that particular guy, are maybe not really meant to be together? If it is meant to be, there is only one way to find out, even if it isn't meant to be, there is still only one way to find out. You aren't going to satisfy yourself by sitting, becoming all jealous, not being able to spend time with that particular friend. Getting your feelings out in the open can only make things better, you will find it out if it is meant to be or not, the girl will either leave that other guy for you, or you will be left to feel stupid meaning that you can live your life, I'd rather my mates gf told me she wasn't feeling the same way and that would make me feel silly so that in a sense my feelings are diminished meaning that I can live my life without conflicting in their relationship, but on the other hand, if I weren't to say anything, my feelings inside my cause gestures to her, or words to her that could sound provokative or flirty.
You did the right thing dan, some may think otherwise but as Becky said, who has a right to deny anyone love? You weren't going into that room with her when you spoke with the intent of leaving with her or splitting her and your friend up, all you wanted was to get your feelings out in the open and any decision on her spliting with your friend or having any kind of intimate contact with you was left to her, it was her decision.
DevilDare makes it sound like you were jealous and wanted to wreck their relationship but you can't help the way you feel. It's better to **** it up, by saying how you feel rather than **** it up without saying anything.
Along time ago I thought my friends current gf was into me (via excessive eye contact etc etc [all the 'right' signals] turns out I was wrong, but I only found that out after I said I liked her, she said it wouldn't be fair on my friend (but didn't deny any return feelings at the time) I said ok, you are right it wouldn't be fair, but don't tell him that I've spoken to you about it, because he will be upset.
She ended up telling him, but I still to this day don't understand why she did as nothing good came of it, me and my friend are still best of mates but it was almost like her intent a the time was to get rid of me from his life, I don't see any other reason why she had to tell him, she didn't do anything wrong, I did, if anyone 'needed' to tell him, it would of been me.
And for some reason now, she doesn't seem to like me, lets just say, if I left her a comment on her facebook status, I wouldn't get a reply. lol
But the point of it is, the fact she was clearly more interested in my friend made me realise that she wasn't the girl for me, which made me instantly accept defeat, the only complication was the fact she told him anyway, I didn't make any physical advances or anything which would warrent her to tell him, lol women, never understand them...