I agree with you; if what she rides in is what she is comfortable and responsive in, there is no reason to 'fix' it; western folks consider a curb bit to be the 'sign' of a finished horse. I do prefer a snaffle, or bosal, but I've had horses who, while they ride fine in the previous types, prefer a curb of some form. If they like it, and respond well in it, who am I to argue, really? The distinction I make is when you "have" to ride in a heavy curb to get a response out of him, and that is when I feel it's time to get some real training in, which will mean going back to something as basic as a snaffle.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."