Exclude the www- append to only one multisite
Unfortunately putting separate htaccess files in each sites folder doesnt work, otherwise it would be quite straight forward.

Perhaps you can use REWRITECOND to do this?

RewriteCond %{REQUEST_URI} yourclientsdomain.com
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]