There have been several threads on this topic and the long and the short of it is no, you'd be better to do a gel overlay on the natural nail and polish over the top.
Or shellac her! That would be the best.
That would depend on the client's definition of "uv topcoat'.
There are 'uv topcoats' that are just polish that protect the acrylic from uv exposure, to keep the colour 'true'.
If she meant 'uv sealant' or a 'uv gloss' that requires a lamp, there are a couple, but I can't remember the brand names off the top of my head.
I regularly use Nailite 'seal-it' uv sealant over my polish frenchs, with no chipping etc.. Everything stays beautiful until the next appointment.
There are times when a gel polish or shellac won't do, and I use regular polish and protect it with Seal-it.
BUT only in the case of frenchs. It's not suitable for a full cover polish as the 'seal-it' needs an etched surface to grab onto.
The frenchs stay PERFECT until their next appointment, and perfectly shiney.
Recently 'seal-it' has been discontinued. I have only 6bottles left. My clients are HEARTBROKEN and I'm now in a scramble to find a uv sealant that performs as well.
Will keep you posted.
PS: SORRY, just re-read your post. Did you mean on nails? OR regular polish on top of enhancements? My bad, what I do is for on top of enhancements, NOT natural nails.