Both of my sisters have terrible Rosacea and Proactive has cleared it up. However, like any skin product, in the first two weeks the problem will get worse before it gets better. It's likely that even if this product works for her in the long term, if she starts now her face will get even worse for prom.
(It is also unnecessary to get the expensive package- the cheapest one does a great job by itself and is no more expensive than what you find locally.)
Alternatively-
Since money is no object, take her to a professional makeup artist the day of the prom. They'll know how to cover it up and she'll love being pampered.