In this case the  server is sending emails but they’re likely being either filtered by the user's spam filter or the user may have mistyped or provided the wrong email address at signup (so their requests to get a new password are going nowhere).

Tip: use the ‘Filter’ at the top of the user manager to search for the user's name to quickly find the user.

Once you bring up his/her account simply change the password manually to something  that isn’t too obvious.

You'll also want to make sure the ‘block user’ checkbox isn’t checked and verify their email address.

Once complete send the user an email letting him/her know what the new password.