I invited my friend to join, but he fail to login to Delta app with Gmail account. He double confirms this gmail account has enabled the IMAP and the password is correct.
Please advise.
hello, welcome to the forum, you protected the email in the “email address” field but you leave it exposed in the error message 
did your friend enabled the “less secure apps” option or generated an app password?
As an option, you can login with Android, it uses OAuth2 and does not require enabling insecure apps. Then import export a backup and import it on Desktop.