I just emailed the host that the hacked site is with hopefully they can sort out this issue for the website owner... giving me back a good linking website hopefully. hehe
A site that is hosted and domain name registered with you has been hacked, you may want to tell the owner of example.com that some hacker is using there site to try to do SEO to pill websites.
The hack is creating hidden pages in the form of
plus thousands of others
which only will show up to seo bots to normal users the homepage will be shown.
It would be a good idea to tell the website owner about this problem and help sort it out.
The website may get penalized by google but if the problem is sorted out and the hacked hidden pages removed creating 404 pages to these pages will mean all bad links pointing to the site that are also will harm your clients site rankings in google will have no more effect and the site should go back to normal and restore its ranking in google and other search engines.
It is a wordpress site so most likely it was a theme or plugin that has been compromised and led the site to be hacked and no doubt there is some bad code in the htaccess file.
It is probably in your best interests to sort out this persons site otherwise they will probably get zero traffic, lose interest in the site and then stop hosting with you...
Well that's my good deed for today.