I agree, you can never be sure, but when Google explain how their alert system works, if that fails then it is a bug report to Google's Chrome team rather than BT or Yahoo, no matter how much the latter two deserve it.
Plus, I and many others followed the recommendations and never got bothered with such alerts. I suspect IE and Edge have the rule enabled by default for non-trusted sites, ie, block alerts.
It seems to me to be a no-brainer to block alerts in Chrome. Irrespective of whether or not the web page requests them, just ignore the request. The latter will still be in the source, and, if Yahoo have removed it BH cannot prove one way or another that it used to be, unless he archives every web request and response he ever makes.
So
1 leave things as they are and have a small chance of saying told you so, or
2 deny requests for alert boxes and have a vanishingly, infinitely small chance of getting another alert. (Sorry, couldn't resist the temptation!)
Which do you choose?
If, on the other hand, BH is rejecting cookies, or viewing his web mail in an in cognito window, it could be that Yahoo does not repeatedly display the dialogue box.