We need more information:
Which web browser is your friend using?
Does the message only appear on one particular website or on most/all of them.
How does it appear? (Is it a pop-up box or does it, for example, appear across the top of page displayed?).
At a wild guess, however, (which is the best I can do with so little information available) I'd suspect some type of malware. Get your friend to download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(When installing it, she should take care to deselect the pre-ticked option to take a free trial of the Premium version).