I had the same problem at school. But my friend was just really naturally flirty where as Im more laid back, when I meet guys I'd rather have a laugh and a joke with them than size them up and decide if they are boyfriend material. Sad thing is I would get close to a guy and along she'd come and flirt them off their feet, dropping them a little later. But now Im the one with an amazing boyfriend and she's never had a long term relationship, HA to her.
Just be yourself, the right guy will come along, just be a bit more confident and don't be afraid to be a bit pushy with her for the attention. I found I'd lurk in my friends shadows just because I couldn't be bothered to fight for the attention. Don't let her make you lose your self confidence, that will not help you in anyway, boost yourself up, go out and be confident. Take the attention away from her. Girls like this are fuelled by male attention, if she wasn't getting the attention she'd be less confident.