How to remove Windows-related malware warning on Mac?

One thing that makes it an obvious scam is the phone number has nothing to do with Microsoft. Just put the number in a search and you'll see it goes nowhere.

There's no such thing as a website or popup from one that can prevent you from simply moving away from the page, or shutting down your Mac, no matter what browser you're using.

Whatever site you're seeing this on is displaying the scam. Stop going there. To clear it from Safari, hold down the Shift key, then launch Safari. That tells Safari not to load any tabs or sites from the previous session.

Secondly, you can stop almost any such garbage from appearing simply by installing an ad blocker, since an ad is essentially what these are. We use Ka-Block! on all of our devices. It's free and available through the App Store. Works just as well as any other ad blocker I've tried and tested - paid or not.

BadTuna wrote:

It a pop up. It wont allow you to simply close it.

A lot of scam Web sites are programmed to hijack browsers like that.

Hopefully you won't run into something like this again. But if you do, and you can't close the window or quit the browser the normal way, select Force Quit… from the Apple () menu and kill the browser.

Then hold diown the SHIFT key and launch the browser again. This tells the application that it should not reopen windows just because they were open when you last quit.

Prevent apps and windows from reopening on Mac - Apple Support

As a final precaution, clear browser history so you don't accidentally select the site that abused you.

Looks like a scam message presented by a Web site.

U. S. Federal Trade Commission – How To Spot, Avoid, and Report Tech Support Scams

"Tech support scammers may try to trick you with a pop-up window that appears on your computer screen. It might look like an error message from your operating system or antivirus software, and it might use logos from trusted companies or websites. The message in the window warns you about a security issue on your computer and tells you to call a phone number to get help.

…

If you get this kind of pop-up window on your computer, don’t call the number. Real security warnings and messages will never ask you to call a phone number."

Microsoft Support – Protect yourself from tech support scams

"Scammers might also initiate contact by displaying fake error messages on websites you visit, displaying support numbers and enticing you to call. They may also put your browser in full screen mode and display pop-up messages that won't go away, apparently locking your browser. These fake error messages aim to scare you into calling their "technical support hotline".

Important: Microsoft error and warning messages never include phone numbers."

Complete and utter Scam.

Even in Windows Defender does not simply disable the computer.

Assuming your wife is not running Windows on her Mac through bootcamp, that's still a scam yes.

When exactly is this popping up? Does it happen when she does something specific? Like open a web browser or visit a website? Is she using a bookmark on her desktop when this happens?

These types of scare-mongering popups are always scams trying to get you to call the scammers and give them your details or payment methods to re-enable the device for a price pretending to be a legitimate company.

As mentioned, if this happens while in Safari, you can force close it form the Apple menu and then hold the shift key when opening it again, so it doesn't load any previous tabs.

You want absolutely no anti-virus software installed on your Mac, including the Microsoft Defender: Security found in the current Mac App Store. No viruses or malware written for Windows can install on macOS and unlike Microsoft, Apple hardened the operating system to keep things from installing into the operating system itself.

This is discussed further in macOS - Security - Apple and here: Apple Platform Security - Apple Support

Some websites allow bad actors to pop dialogs in your browser claiming to have found nn viruses and to call some phone number to get help. This is not from Apple and is always a scam. You can visit Safari Settings > Websites > Pop-up Windows and Block.

The Windows Security screen you have shared is showing a window created on MS Windows, not macOS, so that doesn't make sense unless she is using Windows in a Virtual Machine. On Windows, you will get a Windows Defender anti-virus update daily and for good reason — Windows is a submarine with a screen door for security.

While there is such a thing as Microsoft Defender for Endpoint for Mac, I would seriously doubt you have it installed.

These are very common scam messages. What matters is how you're seeing them, which would determine how you may be able to stop them.

Are these appearing as notifications at the top left of the screen? Are they coming to you as emails or messages? Or are they popups from a web site?

Microsoft Defender is a windows application it does not exist for Mac, so it's kind of strange it's showing up on a Mac and claiming there's an infection it has detected somehow.

When or where exactly is she seeing this?

If it happens when accessing a website, it's simply a scam ad, and nothing much that can be done about it and there is no actual Malware on her Mac.

To prevent popups from various sites try the following: