to go along with what everyone else is saying its always best to take the high road in these situations and do the classy thing.

Reguardless of your feelings toward Rachel I would not say anything negative about her to anyone and respect that the other 2 people want to hang out with her. Let them make judgements about Rachel based on how she treats them, not what you say....if she really is as flaky as you described they'll probably figure her out eventually.

I would also try and be as nice to Rachel as possible while at work...you can be civil and polite it does not mean you have to hang out with her and invite her over for dinner.