You are correct. It is Day/Month/Year in Europe.
My sister in law almost got stranded in the United States when it was time for her to go back to Russia. After she visited us for a week, I took her back to the airport. The front desk wouldn’t take give her a ticket because they thought the passport expired.
The front desk lady read the passport expiration date as Month/Day/Year which would have been expired. So management had to get involved and educate the lady. Luckly, my sister-in-law was able to catch the flight in time.
Sorry to hear that— I’m glad someone got involved who took care of it, and now hopefully that person won’t make the same mistake again. But too bad you and your S-IL were the ones who had to go through the stress of that individual’s training....