Spam Email Sent From My Dad’s Yahoo! Without His Knowledge?

On two separate occasions, I have noticed spam email from my dad in my Gmail inbox. All it says is something like “yourpillsworld.net”. I was on my iPhone, however, I did not click on the link or open the emails, just in case. My dad did not send these emails out. They were addressed to many different random people in his contact list. He checked his sent emails, and all of them had been cleared by someone. Has his email been hacked? Is this a virus? Has this happened to someone else out there? How can I fix this problem so it will not happen in the future?
Thank you!