Has anyone received a security alert directing the iPad user to call MAC customer support?

All such messages are scams presented by criminals. Do not interact with the message or the sender. Do not download anything based on such bogus messages. Close the offending page and avoid sites that are involved in such bogus advertising tactics.

Phony "tech support" / "ransomware" popup… - Apple Community

Scam. Do not click or otherwise interact with the page. Simply close the browser tab.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Providing that you have not attempted to jailbreak your device - or have bypassed protections by side-loading third-Apps (if you don’t know what this is, then don’t worry about it), then it is highly unlikely that your device will have been infected with a virus or other malware.

Most alerts that you see are pop-up messages from websites - these being designed to scare the unwary into giving away sensitive information - or to fool you into doing something that you shouldn’t.

Due to the system architecture of iOS/iPadOS, unless jailbroken, your iPad is not susceptible to traditional malware infection per-se. However, as with all computer systems, there are still vulnerabilities and exploits to which you remain at risk.

Browser-based attacks (such as the scam that you have encountered) can largely be mitigated by installing a good, Content and Ad-blocking product. One of the very best and most respected within the Apple App Store - explicitly designed for iPad, iPhone and Mac - is 1Blocker for Safari.

https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024

1Blocker is highly configurable - and crucially does not rely upon an external proxy-service of dubious provenance. All processing takes place on your device - and contrary to expectations, Safari will run faster and more efficiently. 

Unwanted content is not simply filtered after download (a technique used by basic/inferior products), but instead undesirable embedded content blocked form download. A further benefit on metered services, such as cellular connections where you data may be capped or chargeable, this not only improves speed but also saves you money.

When using a good quality Content blocker, a high proportion of otherwise inescapable risk when using your Safari browser, or linking to external sources from email, is effectively mitigated before it even reaches you.

There are additional protections that can enhance protection further, such as using a security focussed DNS Service in preference to automatic DNS settings. This can either be set on a per-device basis in Settings, or can be set-up on your home Router. I recommend using one of the following services, for which IPv4 ad IPv6 server address are included here:

Quad9 (recommended)

9.9.9.9

149.112.112.112

2620:fe::fe

2620:fe::9

OpenDNS

208.67.222.222

208.67.220.220

2620:0:ccc::2

2620:0:ccd::2

Cloudflare+APNIC

1.1.1.1

1.0.0.1

2606:4700:4700::1111

2606:4700:4700::1001

Use of the above DNS services will help to shield you from “known bad” websites and URLs - and when used alongside 1Blocker, provides defense in depth.

I hope this reassurance and guidance proves to be helpful in resolving any issues with malicious websites.

There is no validity. All such messages are a SCAM. Simply ignore.