What is 'FOND' font association usability?

A yellow caution symbol from Font Book? If so, it's safe to ignore. FB reports a lot of things as caution errors that have no effect on using a font. A red symbol though shouldn't be ignored. Delete the font if you see an error message like that.

There's nothing wrong with the font, that's just what Font Book does.

To explain, it tries to show both the standard character set, and as many Unicode extended glyphs as will fit in the window per the point size you choose. When there are no glyphs in those positions to display, it replaces them with the question mark from the font, LastResort. If you have an old copy of Wingdings active, the entire font displays as question marks in Font Book.

I have no idea why a programming engineer decided using that glyph as a placeholder was a good idea. It only serves to confuse, like here.

Here's one description of the FOND table.

Unless you're developing fonts from scratch, you don't do anything with the FOND data.

You should acquire a new, clean copy. I wouldn't use one that heavily damaged. As one possible solution, ask your client to email you a new copy.