She has completely lost her sheen or its rubbed off in patches? Unfortunately the latter seems to be pretty common and a very easy occurrence.
Over the years I have tried a couple of different pearlised paints and none of them quite have the same transparency even watered down as the original. They do look fantastic though and you can get a great effect if you are doing all of a pony which is completely matt. My problem was always getting it to quite the same transparency and coverage on a pony I was trying to patch the pearl coating back on to - it just never looked 100%. Almost like trying to reapply nail polish to a chipped nail. Once I did magic eraser off the remains of coating on a baity Sea Shimmer to see and the paint was much more even when finished but the magic eraser had left a grainy effect on the coating which was left so she still was not right [should have known
].
Personally unless its really really bad or its a couple of very small areas which you may not notice the patching on, I would be tempted to leave her as is.