Unless your client runs a separate Roundcube installation on his/her domain, independent of the default Roundcube component used by Plesk, it should not matter. However, as long as PHP 8.3 is installed, the Roundcube component from Plesk will run regardless of the PHP version used for the domain of your customer. Because the Roundcube component from Plesk runs independently from any domain PHP settings/configuration.
If that's not the case for you, provide more details so forum users can make suggestions for troubleshooting any issue you might have.