I have to say I don't envy your situation. It's one of those ethical questions you get in interviews, but hope it never actually happens.
Have you tried talking to her and clarifying the situation? Did she totally make stuff up, or did she "exaggerate"? Like avig said, both are bad but let's face it...99% of interviewees exaggerate a tad. Interviewers know that, and it's only when something is completely fabricated or extremely exaggerated that I'd say you need to get involved.
Do you have any evidence to back up your claims?
If she truly did make stuff up which led to her receiving those few key points that differ between offer and a rejection, and you can prove it, then I would do something about it. I'd suggest that she comes clean herself, and if they decide to still give her the offer then good for her. If she doesn't want to come clean I would contact them myself.
Like others said, there is still time for the offer to go to someone who didn't make stuff up, so doing something now can have a positive effect. Yeah, it sucks that she would lose her spot (maybe), but that's a spot she didn't deserve in the first place.