They are probably Asterina stars...and though there are some that rarely appear to eat SPS corals, overall it is very rare. They do not "decide" to eat corals.
the overwhelming majority of hobbyists with LR probably have Asterina stars, and very few have issues. However, people have reported problems with both the coral issue and leaving spots on coralline algae. But there are loads of threads on Asterina stars and you should see if this is what he has.
Overall, I consider asterina stars to be a harmless and cool addition in the tank, and pose no major risk beyond anything like adding hermits or crabs to the tank, as well as bristleworms. In all there are "bad" species that cause problems but overall most do not. So you have to keep an eye out. But I don't tend to worry, myself.
If you intend to keep an SPS reef, you may not have the same view I have.
But DO a search on Asterina to make sure this is what you saw.
