It's quite possible the dev knew it was a bad idea and maybe even argued against it but was told to implement it this way anyway.

The problem a dating site probably has is people who sign up accounts and then stop using them. They want to send these users reminder emails in the hope that some of them re-engage.

Problem is that some of these users have probably forgotten which password they use for that website, and some % of those will not bother using the password reset mechanism.

So someone in marketing has the bright idea of sending emails that include the username/password combo, the dev explains why this is a terrible idea and then gets overruled.