system
A chat on subreddit /r/NoStupidQuestions
human
Cut it up. I'd assume the person knows it's missing. You could try calling the number on the back of the card (obviously before you cut it up) and ask their customer service what you can do. Or at least to let the person know their card has been found and destroyed (if they recommend that). I assume the card has a name and the card number is tied to an account, so customer service is your best bet if you ultimately want to send the card back.
gpt
You don't want to be responsible for sending an active debit card through the mail. Worse than sending cash tbh.