A lot depends on how much white there is. White on a ruby is considered a fault, but all dogs have faults. It's just a matter of what faults the judge sees as more serious. Unfortunately, white is very easy to point at and say "that's a fault" vs. a fault that's a little more subjective. If she is very good in other respects, then she may have a chance at winning. Be aware, though, that you have to be able to take rejection if you're going to show, because you will lose a lot more than you will win.
I wish I could give you a more definative answer, but showing in conformation is very subjective. I will say that a big advantage to showing is that you really can meet some very nice people and none of them will tire of you talking about Cavaliers. And as you show more and people start seeing you more and more at the shows, lots of opportunities open up if you decide you want another show potential dog.