why would it be a casualty loss?   

Your client lives there?  Have you heard the news about casualty losses (since they were mostly taken away in 2017)?  Not to mention, mold is usually not sudden and unexpected.  That's why insurance doesn't cover it.  

Maybe try as a medical expense.  But if the problem was in Unit 26 and you're in Unit 82, it may be difficult to make that case. 

Interesting - thank  you. . .

If it is part of the Chinese gypsum (drywall) mold issue, you have to file a private or class action lawsuit. There is no Governmental support for the issue.