Perils of using a "recycled stamp"

My birthday recently and friend who doesn't live near always sends birthday card but didn't receive one. About a week later I received a post card from
Royal Mail to say they had an item and not enough postage on and to pay £2.50 which I did. A week or so later my birthday card turned up and it was from the friend and couldn't understand why extra postage to pay. On close examination she had reused a stamp as the corner was turning up and brown backing on the stamp! Oh dear I had to send email to her saying thanks for card which arrived today but felt embarrassed to explain the delay!
So at your peril to recycle a stamp! It was one of the bar coded ones and I think must be scanned or something to show been used before. Would you have told your friend?