Do you still see your friend's old email address as a suggested contact in the Mail app? You can get rid of that.
When sending an email, the Mail app uses autofill to suggest a contact based on the name you enter. Sometimes, outdated email addresses show up in the suggestion list, even though you've updated the person's information in your Contacts app. There is a way to remove email addresses from these autofill suggestions so you don't accidentally send something to an unused mail box. Here's how.