"Copy show" feature

What the user is describing isn't a feature called "Copy show" in any official capacity. It sounds like they're referring to anti-copy security features used in sensitive documents like certificates, cheques, and currency.

Copy-evident patterns

Void pantographs

Anti-copy backgrounds

Security screens

I'd highly recommend they contact whoever is printing the certificates to ask for what security features they can offer. 

Can this be done in InDesign?
Sort of.

You can design a subtle background pattern that looks clean when printed but shows a hidden word like "COPY" or "VOID" when photocopied. You'd need to create a textured pattern or screen in InDesign fine lines, dots or a halftone that obscures a word like “COPY” or “VOID” embedded subtly. Use very fine detail almost at the limits of a printer's resolution to make sure scanners and photocopiers can’t reproduce it accurately.

When someone copies it, the pattern breaks down, and the hidden word becomes visible.

BUT  the real magic is in the print process
Real anti-copy features use security inks, microtext, UV-reactive elements, or metallic inks.

Some also use dither patterns that vanish or shift tones when copied.

So yes, you can simulate a basic version in InDesign, but it's not foolproof, it won’t stop high-end counterfeiters, it depends on how it’s printed.