Client is adamant that .notdef glyph must print as a square (!)

I was supplied a PDF of a poem translated from Farsi into English, which contained several .notdef glyphs - squares symbols in between verses.

I looked up other translations of this poem, and an asterisk was in place of the .notdef.

However, the client is adamant that it must be a square in the print file!

I've tried to explain to them that the square is an error/warning symbol and does not really exist in print.

Any advice or pointers for how I can further explain to them that they are mistaken? Or am I mistaken?

In 20 years of typesetting, I've never come across such an issue before with a client...