recruit your friend (the jealous one with the boyfriend) to HELP you get the girl you're after BEFORE you approach the girl.
"Hey, that girl is totally cute. I'm going to go talk to her. Come up to me and hang all over me like you want attention. I'll acknowledge you and be completely friendly, but non-reactive, and tell you that I'll be right over, but first I'm going to finish this conversation."
Use it to your advantage. But don't let her talk much or drag you away. If she tries, politely, lovingly hug her and tell her she's awesome, and that "you'll be with her in a minute," as if to service her, and then send her along her way.
If she does manage to pull you away, be sure to look at the girl you're talking to as if you're in demand. Smile and shrug your shoulders as if to say, "hey, whattya gonna do? I'm hot property." And then be sure to give yourself proximity to the girl later, as if not to notice her, so she has the opportunity to re-open you.