my wife's mac got a warning message from premiummachelp.*** " warning OS crash" due to a 3rd party application

Download and run Malwarebytes Anti-Malware: http://www.adwaremedic.com/index.php

You can also remove the adware manually using the steps provided by Apple: https://support.apple.com/en-us/HT203987

After that, make sure your default search engine is set to the one you always use. To do so, Open Safari - Preferences - Search.

Go to Safari menu (At the very top right side of your screen next to Apple icon), choose "Quit Safari"

press "Shift" button and while holding this button on your keyboard single click on the Safari icon on your Dock.

Open Safari - Preferences - Privacy - Remove All Website Data.

Also, I believe it would be a good idea to install AdBlock.

You likely do NOT have malware/adwre - this is a different critter on websites one visits called "RansomWare" = the website has been hacked or is in on the scheme

Phony "tech support" / "ransomware" popups and web pages < tells about it and how to fix it