It’s a common scenario: you need to send some sensitive information to a colleague or friend, and you know that email is not the way to do it. Instant messaging isn’t any more secure. You could use chat: Skype, WhatsApp, or best of all Signal. But that requires that you already be connected.
Google Drive, Dropbox, or other document sharing services are options as well, but once again it requires that you both have accounts linked to your email addresses, so someone might have to create one.
A simple solution is PrivNote. You can create a note that can only be accessed once, so you know that once the recipient views the text, no one else can get it. And if they cannot view it, you know someone else has and can take appropriate action.