But I’m not reading it in the UK. 
Hear hear! Emily Post said the good hostess never makes the guest feel wrong; if he drinks out of the fingerbowl, the hostess should, too. The hallmark of genialty is making all feel at ease.
While working for the PO-lice we said VIN number. Redundant, I know. (Vehicle Identification Number.) But just plain VIN doesn’t roll off the tongue on the radio. And if your numbers matched up with NCIC’s wanted numbers you may have been grammatically correct in your protests, but you were also busted.
I would gripe about something less prevelant, like bedbugs. Scary, but not nationwide. I suspect the need to add “number” might later be found to be stamped on our DNA.
Saying “VIN number” and “PIN number” might be considered redundant. But it is also good communication, imo. Associating the word “number” with PIN and VIN allows a person to instantaneously know what is being referred to and more readily than saying VIN or PIN alone - and esp “VIN” which when spoken as a word doesn’t easily conjure an immediate image of what one is talking about, but more readily when “number” is added. Esp when speaking over the phone.
I don’t recommend you correct her. But if you insist, then …
Tell her it’s a “PI number”, as in “Please enter your PI number”. In that way we preserve the separate word “number” that so many folks have suggested is valuable usage to set context for the acronym. And we eliminate the hated redundancy.
Now whether “PI” is pronounced like the Greek letter pi or like pee-eye is up to you. But don’t get it wrong. The only thing worse than a pedant is an erroneous pedant.
C’mon folks, we’ll win this fight yet. Then we’ll turn to “ATM machine”. Yeah, right …
Calling them “ATM machines” was near universal when they were introduced in the early 80s.
I agree that nowadays I rarely hear “ATM machine” from anyone but the elderly & the barely-educated.
Due to the disingenuousness, shall we say, of the OP, I’m closing the thread.