The plugin Tell a Friend puts a button on posts for doing what it says.
However clicking on the button brings up a form that links to a URL that the web browser informs that the certificate is unsafe. With the red warning showing I ignored it thinking this is a WordPress plugin it must be safe.
It wasn't safe. Next day my computer wouldn't boot and I had the BSOD:
"Windows could not start because the following file is missing or corrupt: System 32\Drivers\Ntfs.sys"
I had to repair windows xp using the installation disc, it took most of the day and cost money for a techie geek to help me - simply because the repair wouldn't start as it was supposed to: I had a USB keyboard installed and this wasn't recognised at the repair stage. It took 2 hours at £20/hour to figure this out.
I informed the author of the unsafe certificate and added a supplementary question. I have received no answer and have deactivated the plugin as the link is still unsafe.
I would suggest that if the author does not resolve this problem in a timely way then the "Tell A Friend" plugin should be removed by WP admin.